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.
[W]itches and wizards usually turn, in the first instance, to Disillusionment Charms for self-camouflage or concealment. Albus Dumbledore was known to be able to perform a Disillusionment Charm so powerful as to render himself invisible without the need for a Cloak.
Tales of Beedle the Bard - page 97 - Bloomsbury - The Tale of the Three Brothers
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:
With a snap, the man in the shadows closed the door behind them.
A mass of filthy, matted hair hung to his elbows. If eyes hadn’t been shining out of the deep, dark sockets, he might have been a corpse. The waxy skin was stretched so tightly over the bones of his face, it looked like a skull. His yellow teeth were bared in a grin. It was Sirius Black.
Prisoner of Azkaban - page 248 - Bloomsbury - chapter 17, Cat, Rat, and Dog
And Peter Pettigrew, also 33-34, didn't fare much better, living as a rat for over twelve years:
He was a very short man, hardly taller than Harry and Hermione. His thin, colourless hair was unkempt and there was a large bald patch on top. He had the shrunken appearance of a plump man who had lost a lot of weight in a short time. His skin looked grubby, almost like Scabbers’s fur, and something of the rat lingered around his pointed nose, his very small, watery eyes.
Prisoner of Azkaban - page 269 - Bloomsbury - chapter 19, The Servant of Lord Voldemort
Albus Dumbledore apparently looked very old:
[Dumbledore] was tall, thin and very old, judging by the silver of his hair and beard, which were both long enough to tuck into his belt.
Philosopher's Stone - page 12 - Bloomsbury - chapter 1, The Boy Who Lived
Draco Malfoy showed normal signs of aging in the epilogue when he would have been approximately 37 years old:
Draco Malfoy was standing there with his wife and son, a dark coat buttoned up to his throat. His hair was receding somewhat, which emphasised the pointed chin.
Deathly Hallows - page 605 - Bloomsbury - Epilogue, Nineteen Years Later
And, finally, after death, it seems a bit of youth and vitality returns to the spirit:
Sirius was tall and handsome, and younger by far than Harry had seen him in life. He loped with an easy grace, his hands in his pockets and a grin on his face.
Lupin was younger too, and much less shabby, and his hair was thicker and darker. He looked happy to be back in this familiar place, scene of so many adolescent wanderings.
Deathly Hallows - page 560 - Bloomsbury - chapter 34, The Forest Again
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:
In Harry Potter, #1, wizards don't meddle in Muggle world in the first place. So the problem doesn't exist per se.
In Sabrina the totally irresponsibe witch, they seem to move place to place periodically (same as in Bewitched) - the "Highlander" solution seems the easiest.
On top of that, in such universes, the magic users have near-unlimited magical power and can nose-wrinkle any bad thoughts out of civilians' heads.