At the end of Revenge of the Sith, Obi-wan Kenobi, played by Ewan MacGregor, is present for the births of Luke and Leia, and he appears to be approximately thirty-five years old, give or take. When we again see Obi-wan, he is quite an older man, perhaps in his late sixties or early seventies, played by Sir Alec Guinness. Yet Luke, if the reading I've done is correct, is approximately nineteen years old at the beginning of A New Hope. If this is incorrect, by all means correct me, but if it's true, why would Obi-wan have aged so much?
In-Universe, no real canon answer I know of, but he was a man living in fairly poor conditions (backwater crap of a planet, with really poor infrastructure, and he lived in a cave in a boonies to boot), who had major psychological blows (Leaving aside any war-related PTSD, he raised the damned Darth Vader! And everything he ever held dear was destroyed, along with most of his friends).
Oh, and he has responsibility to watch over the pretty much only hope of the Galaxy, and is fully stymied in raising him properly due to Owen Lars' intransigence.
Having been in a real military, I can attest that fighting a two decade long war could age a man.
going from the Star Wars Wiki, Luke was born in 19BBY, Obi-Wan in 57BBY and Anakin in 41.9BBY, IV takes place in 0BBY therefore it can be assumed that Luke is 19, Obi-Wan is 57, and Anakin is 41 at those times.
Alec Guinness was about 62 during the production of IV, only 5 years older than his character thus suggesting that he some what ages normally
Obi-Wan believed that Tatooine affected his aging process.
From the canonical novel Ahsoka,
He stood up, his knees creaking in a rather alarming fashion. Surely he wasn't that old yet. It must be the desert climate that affected him strangely.
Obi-Wan's wrinkles and bleached hair could be explained by living twenty years out of doors (in a cave, but still) in windy, dry conditions and under double suns. Being homeless is apparently hell on the skin.
The "problem" is that the story was changed multiple times from the original STAR WARS to REVENGE OF THE SITH.
In STAR WARS, Obi-Wan is actually in his early 60s. Father Skywalker was in his late 50s. Darth Vader was much younger.
In the second draft of EMPIRE STRIKE BACK, George Lucas decided that Vader was now Skywalker. Age wise this worked... until the prequels.
In the original STAR WARS, Luke is 21 years old (Leia is 18 by the way). That means that in REVENGE OF THE SITH, Obi-Wan would have been in his early 40s while Anakin late 30s. In fact in REVENGE OF THE SITH, Lucas said that Anakin is in his mid 20s while Ben late 20s.
So now it doesn't work... but then Lucas changed so many things through the years that it is bound to happen.
But its actually simple as there are three ways to look at STAR WARS:
STAR WARS the original 1977 film The Adventures of Luke Skywalker (from which it was called up until a few months before the release)
STAR WARS the original trilogy Its only later that Lucas added in the EPISODE IV title and said that there would be 9 films (which was never the part of the original plan).
STAR WARS the six-part story The story of Anakin Skywalker, as Lucas so wants us to think of it now.
I won't go into details as to all that changed... maybe some other time.
I read something about Owen Lars that explained that extensive exposure to Tatooine's twin suns had prematurely aged his body, so maybe that could be used as an excuse for Obi-Wan's appearance in episode IV. Alec Guinness was 63 years old during the filming of A New Hope, which only makes a difference of 6 years between him and the character, it's not that much of a difference. Still though, I know what you mean, Alec Guinness look like he's in his 70's in Episode IV.
I did some research on this, and gleaned the best answer from here:
Obi-wan is 25 in Episode I. Episode II takes place 10 years later, and Episode III takes place 3 years after that, making him 38 during Revenge of the Sith.
According to the post above mine, Luke was 21 in Episode IV, and I think it makes sense to assume that this didn't change. So, in A New Hope, Obi-wan was 59, and had been living in the harsh conditions of Tatooine for 21 years, which would easily have added a good ten years to his appearance.
Anakin was 9 at the beginning of Episode I, making him 19 in Episode II and 22 in Episode III. In Episode IV, he would be 43. Episode V took place 3 years later, and VI 1 year after that, making Vader 47 at the time of his death. Due to his terrible burns, living in a mechanical suit, and using the dark side for so long, he appears older.
Obi-Wan was twenty-five years old in "THE PHANTOM MENACE". He was thirty-five in "ATTACK OF THE CLONES". He was thirty-eight in "REVENGE OF THE SITH". And he was fifty-seven in "A NEW HOPE". Alec Guinness was sixty-two when "A NEW HOPE" was filmed 1976. The movie was released the following year. This means Guinness was portraying a character that was five years younger then himself.
There is nothing to get excited about.
By the way, Luke and Leia were 19 years old in "A NEW HOPE".
Obviously, when the original 1977 Star Wars film was released, Obi-Wan was intended to be much older than the prequels reveal him to be. "That's a name I've not heard in a long time, a long time.", "Shouldnt he be dead by now?" and "I'm getting far too old for this sort of thing".
Also, 35 year old men don't usually refer to 57 year old men as "Old fossils". I also don't know why Luke's age was changed to 19. Books, comics, trading cards and toy packaging all referred to 20 year old Luke Skywalker back in the day.
You are living on a planet with two suns and harsh environmental conditions. Look at how aged Owen and Beru were. I think it is simply a product of the environment and not exactly having access to treatments that would reduce signs of aging.