Take Dumbledore, for example, who was over 100 (115 or 150) and looked to be only in his mid-sixties or early seventies. Say a Muggle-raised wizard, for example, who is actively a part of the Muggle world looks younger than their actual age (maybe they're eighty years old, but look fifty), how would this be accounted for or explained to Muggles? What methods would wizards use to conceal the difference between their actual age and their younger appearance? Would they have to move around a lot to not be detected?
TL;DR: There is no canon evidence that states that because wizards and witches have longer lifespans that they necessarily look younger at a given age than their Muggle counterparts would. Wizards seem to age at about the same rate as Muggles; they just live longer.
I agree with DVK, who said it first, that this seems to be an inapplicable problem in the wizarding world because wizards and witches don't typically go back and live in the Muggle world. Some exceptions? Snape -- his home outside of Hogwarts was in Spinner's End, which was a Muggle area. Arabella Figg -- she lived on Privet Drive to keep an eye on Harry for Dumbledore; of course, Arabella Figg is a Squib and it wasn't uncommon for Squibs to pushed out into Muggle society to live. There is no information available as to whether Squibs also have a longer life span like wizards do, since they're lacking magic. Godric's Hollow was a mixed wizarding/Muggle village; there was one other mixed village whose name is escaping me. I think living in a wizarding community is the first line of defence against a witch or wizard being regarded with suspicion for appearing younger than their actual age.
In Tales of Beedle the Bard, disillusionment charms are discussed, which render a wizard invisible.
Witches and wizards could simply perform a disillusionment charm on themselves and disappear from prying Muggle eyes.
There are some examples, though, of characters who look older than their ages. Remus Lupin was described as having quite greying hair and a worn, weathered look about him, although he would've been around 33-34 years old at the time of Prisoner of Azkaban. Azkaban aged Bellatrix Lestrange, giving her a hollowed out, gaunt appearance, which is no typically associated with youth. I would postulate that Azkaban altered Sirius Black's, also 33-34 years old, appearance and not for the better:
And Peter Pettigrew, also 33-34, didn't fare much better, living as a rat for over twelve years:
Albus Dumbledore apparently looked very old:
Draco Malfoy showed normal signs of aging in the epilogue when he would have been approximately 37 years old:
And, finally, after death, it seems a bit of youth and vitality returns to the spirit:
So, in sum, witches and wizards don't really need to hide themselves per se (I'm sure there could always be an exception to this here or there) because it's not that they look younger than their age, it's that they have longer lifespans. This is purely anecdotal, but it seems there comes an age where the appearance stops changing so readily. For example, my grandmother lived to be 103 years old and quite honestly she looked the same to me almost all of my life. There came a point where although she was aging, she didn't look any different. I can't find anywhere in canon that says Dumbledore looks like he's only 60 or 70 -- to the contrary, Dumbledore is described as looking "very old" in Philosopher's Stone.
This depends on the work in question: