Specific Context Answer: Currently, the main Comic Book continuity Earth 616 Peter Parker is a young adult; as of this writing (2020), he is portrayed as no older than 31, more than likely closer to 28. This is just an estimation: giving a canonical age will be difficult, due to the nature of comics, and a sliding time scale.
To be fair, "aging" in comics is subjective; not only are there various versions/universes of the Same character, but ret-cons and reboots often occur, starting the same story "over again" fresh with the same version, just in a different decade. More than that, though aging is addressed in comics, its rarely consistent, so in general, most heroes will be around for literal Decades, but only age a decade at most.
The same is true in many works of fiction, especially when "Reboots" are done for the purpose of introducing a new audience to a character for that newer generation.
NOTE: The use of Sliding Time Scales/Time Lines to keep things/characters "current" are very common in fiction, and especially so in comic books.
The simple goal of keeping people in touch with characters they know means that, for the most part, characters rarely change too much. However, as "real world time" marches on, stories for these characters have to be consequently updated. This is the media essentially "keeping with the changing times", in terms of tastes, thematic relevance, aesthetics/looks and registering with the audience so that the book/show/character continues to sell. While there are some characters that age according to normal chronological time, and their styles and stories reflect this, it's rare; Savage Dragon is one such character, and Dr. Who is another....though "The Doctor" is something of a special case, as he acknolwedges that there are "several time-uneccumbered versions" of himself with different ages, faces, personalities, characters and looks.
So what usually happens is that a specific time period (most often reflected by a decade) of a story becomes "fluid"; canon adventures and occurances still happen to the characters and months or years will pass in between events, but the actual date of when a given event happens shifts, usually coming in later and later in the personal history as the "event date itself" is moved forward or updated. This constant slipping forward will consequently keep the character at relatively the same age, while simultaneously elongating/updating the story's life, even as more in-story "events" are piled on to its continuity.
Example: You know how The Simpsons have been a series for over 30 years, but its the same exact story and characters ....even though nobody ever ages?! It's exactly like that; the show's continuity "slides forth", so that the characters always remain the same age, even as more and more events happen. However, to justify this seeming "aglessness", continuous ret-cons will occur that "tweak" the history just a little, so that the Simpsons all being their current ages makes sense. The caveat is this history-tweaking has to work without altering certain canon happenings, effecting major story beats (Sideshow Bob hating Bart, for example) or changing current realities.
For a Simpsons-specific example...nobody can deny that Bart exists, right? Well, for that to happen, Homer and Marge have to meet and fall in love; that's just an immutable part of Simpsons canon. The canon is written in stone; at some interval in time, 10 years before the character we know as Bart is "here" in the "now," his parents have to concieve him. However, whether that's the 1980s, the 1990s, the 2000s or later depends; Bart will always be 10, most likely, but if he's 10 in the 2000s, that means even though, in a meta sense, the actual show started in the 80s, the "chronological canon" would mean he was actually "born" only 10 years before the current decade he is in.
In show, this means that chronological events all happened, (Homer meeting Marge, them falling love, Marge getting pregnant, Homer starting work at the Power Plant, etc) but they all will then seemed to have "happened" at ever later decades. This is exactly why there are no less than 3 stories of how Homer and Marge got together in their "Youth" in high school... once in the 70s, once in the 80s, and once in the late 90s. Essentially, their personal history didn't "rewrite itself"...it just "slipped forward" a bit, so that everything happened, just a a later date, and thus their current ages make sense.
This is the effect that makes judging Peter Parker's current comic book age so damned difficult. I've been a Spider-fan since childhood, and my first deep association with him was the early 1980s cartoon; in this series, Peter was a 19-21 year old college student. By that account, Spider-man has been a "young adult" for at least 30+ years.
And Spider-man, as a character, had already been around for more than 20 years by that point; if he was to age naturally from the time he was introduced in the comics (1960s) Peter would actually be more than 60 years old by now.
Not exactly the prime age for a "kid proxy" or "Relatable" hero most kids are going to follow or emulate (accept maybe Super Gran.)
So to focus this answer a bit more, it would come across as such:
"This is Peter's cannon age in the CURRENT comics"... which again, puts him at late 20s to early 30s, at most.
Oddly enough, there is sufficient evidence for this age estimation, if we follow Peter's canon timeline, even accounting for ret-cons and sliding. Mind you, we're not speaking of the comic book "Chronology" here....as in the linier years of when the books were published.... but the meta "canon chronology"... the verified events that Peter himself has made reference to as "actually having happened to him" in his personal life which mark or influenced his existence.
The best way we can do that isn't by accounting for his comic events (which could be seen to be placed on a sliding time line) but by the interactions/Hall mark life events that have occurred over the characters 60 year history, and which have essentially influenced his canon at certain ages, and thus cannot be denied. This way we can mark establish "points" , guess his age based on the amount of accumulated events at a givem time (more happenings would, of course, mean more time has passed), and then "best gues-timate" how much time passed until the next "big event/person" Peter encounters.
Spiderman himself has, on occasion, referenced that he has been Spiderman for a number of years
Again, this can vary from work to work, and given interpretations. Put that alongside "alternate universes" and the matter gets even more complex (the Ultimates Marvel 1610 universe' Peter Parker, for example, is roughly a full ten years younger than our "main" 616 Universe incarnation.) Our version, however, has indicated he's a grown adult with a long-standing crime fighting career:
This panel as taken from Marvels Avengers: Civil War storyline, which occurred back in 2006-2007. Spider-man publicly revealed his identity, to show his solidarity with Tony Stark's "Pro registration" stance. It should be noted that, at this point in Peter's history, he had been married to Mary Jane for years and they had lost their first child. Considering that she was not his first long-term adult relationship by a long shot (more on that later) Peter's in-story age at this point would likely be mid-to-late 20s. It is assumed that, in story currently, he hasn't aged much from this point and the whole "Civil War" debackle occurred only a few years before....
One of the best "base-line" examples I can give here would probably be Peter's friendship with Eugene "Flash" Thompson. This is because Flash has been a staple in Peter's life since they were both teenagers, and even through University. As such, they are both the same age, and we can account for how "comic time" in years have affected both.
Flash started out as the bully Peter had to put up with in High School (before Pete humiliated him with a display of Spider Power at some point in most stories, or Flash showed a little decency to the guy after Peter's Uncle Ben was murdered). They had an enmity through their teen years, and then grew into a sort of "rivalry" for the affections of different ladies. The two wouldn't become actual close friends until their later university years, which was young adulthood:
This actual panel is from an Amazing Spider Man comic made in the 1970s. By this point, if he was aging according to "real time", Peter should be in his late 20s at least. However, it's indicated in-story that both men are still just in their early 20s. Flash, who is the same age as Pete, says he was in the Army for a while; considering he was with Peter at ESU for a bit right out of High school (18-19), and the earliest he could have enlisted was age 18, if he did perhaps a 2 year stint, this could put them both... at oldest... 21-22.
Speaking of "various ladies".... another marker of his "young adult" age was that Peter was having various girlfriends at this point. And not just a casual date or "one college sweetheart"; though he's more or less monogamous when attached, Peter Parker dated quite a bit in college, spending a lot of time with a several distinct women...
Keep in mind: Dude was Still in University on his first 4-yer degree when these ladies shown above were in the mix! And this isn't even half his total romantic history!
That brings us to both a major significant marker, and a reason why most media outside of canon comics try to keep Peter young, or like to "de-age" him...
"Adult" Relationships can Mature a character in fiction
This can be especially true in comics. Even with a sliding time scale, it can be a difficult balance for writer's to try to keep a character relatable for older fans, while also making the same character enticing for newer readers. As audiences age, many want the heroes they identify with to "Grow up with them", and thus reflect changing tastes from candy and video games to dating and romance. One of Spider-Man's most defining romances, as well as important relationships in comics over all, was his long-time love affair with his first major girlfriend, Gwen Stacy.
Stacy was a central character, long term relationship, a genuine sweetheart, and balancing element in Peter's life for several years while he was in college. She was introduced in 1965, and remained his most central love interest for 8 years; at one point, Peter was going to propose to her. This would have changed the overall Spider Man dynamic and tone of the book; istead of a "care free" and "young" super dude, Peter would likely be portrayed as a more "grown up" hero dealing with marriage and other such issues on top of toppling super criminals.
Yet all this would come to an abrupt end when, for better or worse, Gwen was killed off (basically, FOR GOOD!) back in 1973.
Canonically, Gwen's death has been something that haunts Peter to this very day, even through his marriage with Mary Jane; not only did it change the course of his life, from altering priorities and not being a husband earlier, but it was the first time EVER an earnest hero had failed to save his loved one from doom. In a meta sense, it made comics more mature and dealt with a proper "adult" theme; many say it signaled the end of the "Silver Age" of comics.
In a continuity sense, this was a loss Peter had to suffer as an early 20-something which he never really came back from. It also defined him because it happened at that age, when Pete was still relatively young; everything else in is life afterwards took an adult turn, but few things amounted to such a grievous and personal loss.
This is another reason modern non canon comics and other media interpretations just don't let Peter age:
1. This is kinda "heavy stuff" for a kids' show, and wouldn't be easy to tackle. This is why many of the Spider Man cartoon series, most of which came after this event, have basically never touched it .
2. Given that this young college couple was contemplating marriage and a life together, the loss was especially potent. A teenage Peter wouldn't likely yet have that depth of a relationship with Gwen, so the death couldn't resonant the same.
3. If any of the shows were to age Peter up, they would eventually have to tell this story; just like Uncle Ben's passing, it's very central to Spider Man canon.
Note: To be fair, the 1994 animated series did a version of this, but with significant changes; Peter was an adult, but instead of Gwen, it was the better known love interest of Mary Jane. And she didn't die, but "disappeared" through a dimensional hole.
Gwen's death was something that helped defined who Peter is as a person for years after; this is not something that comes with youth, but only with experience. Even late into adulthood, and being happily married to a good woman he loves dearly, he has struggled coming to terms with it, both as a failure as a hero/protector, and as a personal tragedy. Comics into the 2000s and beyond still show Peter "trying to make sense" of this loss and grieving for Gwen, with the man himself saying "it's been years" since it happened...
The problem of addressing this key story element is solved by one of two ways; either
- keeping Peter young, so that he doesn't come to the tragedy yet, or
- side-stepping it entirely by only following canon "loosely".
Most modern media does a combination of these approaches. And since TV and movies tend to be more widely seen than the comic book source material, this is yet another reason why Peter tends to perennially remain more "Peter Pan" than canon "Peter Parker".
Adults can have A LOT of relationships
And Peter is NO exception! Matter of fact.... he's an UBER example!
Just to lighten things up after that whole "Gwen Stacy" section, we should take a look at the other defining relationships (and a few casual ones) in adult-Peter Parker's life. And for a guy who started off as a total geek and science nerd who was ignored by girls mostly and couldn't get a date to save his life in high school... he certainly made up for it in University and Post Grad years!
By this, I mean that, while he is often portrayed as a bit of a wallflower as a teen, adult Peter Parker rivals only DC's Dick Grayson for the title of "Teen Hero-Turned-Chick Magnet".... and that is saying something!
Mind you, while "cooler", slightly older heroes like Dick's mentor Batman and Pete's own mentor/buddy Tony Stark likely have better overall "numbers", make no mistake; just like his dynamic duo DC counterpart, even after Gwen's passing, Peter Parker definitely gets laid ....
Like ... a LOT!
As in, Even if he doesn't bed them, Guy still somehow catches Female interest he doesn't even know about and get's women WAY out of his league WANTING to bed him a lot .....
As in "DAMN, dude, just FORM YOUR OWN HAREM already!" kinda thing!
Now----- all jokes aside------ the real significance of these adult liaisons to this specific question is this:
1. All of them are canon
2. Most of them occurred within established relationships.... some which have lasted years.
Even if we take it that he started from his very early university days, at which he was...let's say 18, if not 19, 20.... Chronologically, Peter has had some lengthy relationships, which (even in-story) took a lot of time... time in which he, and the girl in question, would be aging. The three most significant ----- or at least, most notable, because of their length/duration, if not impact on him personally---- would likely be Gwen Stacy, Felicia Hardy/The Black Cat, and Mary Jane Watson-Parker.:
Gwen as stated, was his college Sweetheart, and first big love. In "comic time," they were seeing each other (by most accounts) for more than 2 years, almost 3, give or take some drama (she left for Paris for about 6 months at one point); if they started at 18 or 19, that puts him at (youngest) 21 ( maybe 20) when she dies. Not only is the time itself a factor, but how this effects Peter's future relationships will be quite significant also. According to the comics, Peter didn't really get "Serious-serious" with another girl for some time; he was somewhat afraid that what had happened to Gwen (due to him being Spider Man) would happen again... not to mention that he mourned her for quite a while. How much time precisely passed in the comics has never been pinpointed. What is known is that at some point, he began "dating casually" before truly picking up with his next major lover...
Although Mary Jane was around during his time with Gwen (they dated on and off), Peter didn't become intimately involved with MJ until sometime later. In truth, neither were ready for more after the shock of Gwen's death.
As such, it would be Felicia Hardy/ Black Cat who became is main lady mainstay at this point; their relationship was passionate, intense, volatile, hot, and carried over into their civilian personas. Outside comicbook time, the Black Cat was first introduced as an "ambiguous villian" character in the late 1970s. She became a romantic fixture in Spidey's life in the 80s, wanting to fight crime by Peter's side and turn over a new leaf from being a cat burglar to become more of a "sometimes heroine/anti-hero". This change of heart was thanks to Peter's inspiration; he was the first truly "good" man she felt she could totally trust.
Peter first realized he had feelings for Black Cat when she almost died trying to protect him in 1979; "Cat" as he affectionately calls her, remained an "On Again/Off Again" love interest even after Peter got his black symbiote costume in the midst of the Secret Wars, which happened in 1985. By most accounts of comic time, they were purportedly on this romantic rollercoaster for a year or more; keep in mind, this was after Gwen's death, so they didn't get together immediately. Even if we low ball it and say this happened.... let's be generous and say 6-18 months after Gwen's death, to give Peter a fair mourning period (which his sensitive nature would likely take), and accounting for the 5.5 year "real time" difference in Gwen's death up to then... Peter would still be 22-23 at his youngest when they finally broke up.
- Mary Jane Watson is often recognized as Peter's "End game True Love," having helped him heal somewhat from the loss of their mutual friend//his long time girlfriend, Gwen. Chronologically, in "real time", that death happened a decade or more before in the 1970s; they began dating again a "bit later" in the comics (actually 5-10 years, real time) and became serious after Peter came back from the Secret Wars. Their relationship had a LOT of ups and downs, from miscommunications, to emotional showdowns to near break ups and different big life events which took them away from each other....however, Peter would end up proposing.
Adulting and Marriage years
Although they feared they were "marrying too young", indicating they were not too long out of university and likely still in the "early" 20s, Pete and MJ would officially tie the knot in 1987 in The Amazing Spiderman Annual #21. They remained happily married--- if with problems and a few notable "Super hero life stresses"---- up until the much maligned One More Day story arch, where they essentially traded their "love" (all their years of marriage, not the shared emotional bond) to the demon Mephisto to save Aunt May's life in 2007. Given that Cat had moved to Paris by 1985/86 to start a new life, and Peter and MJ didn't "jump" into marriage but were still young, they were each likely around 24 years old... probably closer to 25. And by that time, they had been friends for almost 10 years, having met just before Peter entered university, or in his senior year in highschool, via Aunt May.
It is nearly impossible to determine how many "comic years" the Parkers were married for with any kind of certainty... but a safe estimation, given the amount of events they went through in their marriage, just personal and professional, would be "more than 5, but less than 10". So, if we average it out to maybe 5.5 years married (low ball), and presume they were both 23-24 ( again, low ball) when they jumped the broom, then as of One More Day, they were both around 28 or 29....more than likely, closer to 30.
Even if we eliminate all the other romantic drama in young Mr Parker's life, these three relationships by themselves would put Peter from 19 to almost 30! That's 10 years of adult drama spread over a roughly 42 year "real time" period from the time he started dating Gwen, til his divorce from Mary Jane(1965-2007)...and we haven't even gotten to his current iteration, yet!
The whole reason One More Day was instituted was that Marvel wanted to inject a sense of "freshness" into the Spiderman titles, and they thought the best way to do that was by making him an "unencumbered bachelor" again. As stated before, mature relationships can age a character, and adding "Adult themed romantic issues" on a character gives them a completely different feel and age to "Teen romance". This is the reason there's a difference in feel and portrayal between the adult 616 Peter and Mary Jane's relationship...
...and the "Teen butterflies" romance of Spiderman Loves Mary Jane and other teenage incarnations like Ultimate Spiderman (both the cartoon, and the admittedly darker comics).
Moving on, both figuratively and chronologically, from Peter's most definitive romance, we then come to the Superior Spider-Man stage. This was when Doctor Octopus' mind was inhabiting Peter's body, and ran from 2013-2014. It should be noted, however, that (canonically) a full year Did NOT pass in the comics at this point during Otto's time as Peter.
While essentially wearing his body and suppressing his consciousness, Otto Octavius does , in truth, an admirable job organizing Peter's life, to a degree; he finishes Peter's doctorate, starts Parker Industries, discovers ways to maximizes Peter's fighting potential, and even manages to put a dent in city crime. While doing all this, Peter/Doc Oc also starts a rather sweet, adult romance with a lovely woman called Anna Maria Marconi....
This was no "fly by night" dalliance or "interesting diversion", either; it seems Otto (in Peter's body) had genuine feelings for this woman! So much so that they had even moved in together. To my knowledge, he didn't really reveal his identity (as Dr. Octopus, not Spider Man) to her. Naturally, when Peter got his body back, this made things a bit difficult, especially since the man Anna Maria loved and had been living with was essentially "not the same person" anymore....
This created further complication (and no end of hilarity) when a young woman empowered by the exact same spider that bit Peter, Cindy Moon, a.k.a. Silk, became something of a fixture in Peter's life. This was not only because Silk is implied to be another significant relationship for Peter, but because of their shared biological and "Spider-Totem" origin.
Essentially, on a physical and metaphysical level, Peter and Cindy are very much "made for" or "keyed to" each other. One of the effects of this is that they consistently give off natural pheromones that incessantly drives them to RAVENOUSLY MATE with one another, regardless of time, place OR other people involved! This damn-near instinctual "Can't Fight it Spider Sex animal impetus" is unconscious and occurs if they are within 20 feet of one another, so.... well, stuff like THIS happens:
Even if we are to assume only a year to 18 months has passed from OMD to the time of Superior Spider Man, that still puts Peter...at his youngest...... at between 29-31. And given everything that happened in between (Spider Geddon, Into the Spider Verse, Dead No More, War of the Spider Men, 2015 Secret Wars, House of M, House of X, the Atlantean Invasion of Wakanda, just to name a few).... it's kind of hard to justify how "only a year" could have viably passed.
Final Answer: Given all this accumulated evidence, by most accounts, the "main" continuity Earth 616 Spider man/Peter Parker should be in his early 30s in comics. At the very least he is a young adult, not yet close to middle age, and--- given room for error here, and events happening a bit before--- an extremely generous "best guess" would place him in his very late 20s if we are being extra conservative. As to why he doesn't age, or at the least seems to do so extremely slowly .... we put that down to the comic books sliding time line.
Again, this is just an educated estimation, and the comics themselves could prove otherwise, but until Marvel gives some official details, this is the best we can run with based around available evidence and supposition.