Magneto, despite having white hair for most of his adult life, is actually physically in his late thirties/early forties, thanks to being reverted to infancy by Alpha the Ultimate Mutant one of the many times he was Absolutely Finally Defeated, then later re-aged to his physical prime by the Shi'ar agent Eric the Red after the series returned under Chris Claremont. Neither of these events get mentioned much anymore, though. The movies have him the age he'd actually be.
Logan's son Daken counts (while Wolverine himself, being born in the 19th century, just tips the scale at Really 700 Years Old). Daken was born in 1946, which makes him old enough to be the father of most of his teammates when he served with the Dark Avengers. He barely looks like he's in his early 20s most of the time.
Blade was born in 1929 and barely looks out of his mid 30s due to being half vampire.
Nick Fury was a World War II combat sergeant and hero, heading up the "Howling Commandos". By the time he became "Agent of S.H.I.E.L.D." in the 60s, he was taking a special "youth formula" to explain why he had barely aged a day since then without some outlandish retcon. Not that that youth formula isn't any weirder.
Parodied with Nextwave's "Dirk Anger", who was 90 years old and kept alive with drugs and pureed live chickens.
Nick Fury is completely removed from World War II in the Marvel Cinematic Universe, with Captain America taking his place as the leader of the Howling Commandos.
Apparently, Nick's youth formula is starting to malfunction, leading him to retire. Even if it wasn't, he gave the last of it to Bucky Barnes when the latter was suffering from a massive injury. This also means that Bucky will retain his age for a while as well. Even more notable since Bucky is already Older Than He Looks, having been kept in cryo between missions after WWII. He was a teenager during WWII, and is portrayed as a man in his late twenties in modern times, but he's actually at least 80 years old by this point.
Captain America himself, obviously. The man was at least in his late 20s when he was frozen at the end of WWII, and in modern times, is depicted as being probably mid-thirties. He's actualyl close to 100-years-old.
The Punisher. Frank Castle is heavy into this trope, mainly because the character is so firmly tied to the Vietnam War. In the comics involving the mainstream Marvel Universe, he's generally depicted as mid-to-late 30's, and each writer who has the character for any length of time will invariably mention his fanatical fitness regime and adherence to a balanced diet. Nick Fury has the Infinity Formula, and both Cap and Bucky were frozen for decades, but Frank has been active and kicking ass since his introduction. The last American troops left Vietnam in 1975, so even a 18-year-old soldier who was there only briefly would be 58 as of 2015. Frank's background, however, involves multiple tours and coming home to a wife, as well as two children who were well out of the toddler stage. When Frank gets together with men he knew during the war, they are depicted as having aged appropriately and even commenting on Franks' youthful appearance. "Frank, what is it with you? You don't look a day over 45!"
It's actually becoming an issue in the mainstream universe, as writers are aware of the conundrum and try to avoid mentioning his Vietnam service, and instead just referring to him as having extensive military experience.
The MAX series depicted Frank as the 50-60 year old man that he was, who relied on massive firepower and methodical planning to kill his opponents, but was nevertheless still extremely dangerous in hand to hand combat. The final (and extremely well-done) MAX story showed Frank's age (and mental problems) finally catching up with him, although he stayed alive and kicked ass long enough to get the job done.
Mystique is self-admittedly at least eighty years old, but her Shapeshifting powers greatly mask this fact.
Black Widow also has been handwaved as being experimented on with something to keep her young. It explains how she could be a spy during the Cold War and still not look a day over 30.
Similarly to the Nick Fury example above, the Marvel Cinematic Universe sidesteps the issue altogether by making Black Widow an ex-mercenary who just happens to be from Russia, rather than a former Soviet spy.
Tony Stark: You have a quiet reserve. I don't know, you have an old soul.
The League of Extraordinary Gentlemen has Mina Murray, who's immortal due to her bath in the fires of youth at the lost city of Kor (from She: A History Of Adventure), and thus always looks to be in her twenties. This might lead to her becoming Really 700 Years Old after awhile but, at the time of the Victorian-era comic book, it just means she looks really good for her age.
In Planetary, there are certain characters called "century babies", that are born at midnight of a century's initial year(i.e. 12:00 a.m. January 1, 1900, etc.). These characters can still be killed, but once they reach a certain point in adulthood, they stop aging and it's never made clear if they can in fact die of old age. Century baby Elijah Snow looks about 40 but is(at the time of the book) in his late 90's; Jakita Wagner and Anna Hark are the daughters of century babies(Lord Blackstock and Hark, respectively); both are in their mid-70's and neither look a day over 30, with Anna predicting that she will live for at least three hundred years. Spy John Stone has been aging at about 1/5 normal speed since his 40s. Then there's AxelBrass, who figured out how to stop his aging when he was 43. Jenny Sparks from The Authority is also a "century baby" and stops physically aging at the age of 19, though she looks to be in her mid-20's.
Jack Knight from Starman looked like he was somewhere in his late 20s, when he was really in his mid 30s. It's not that drastic, but they do lampshade it a few times.
Most of the surviving Golden Age DC characters have had a few decades taken off as a result of various adventures and continuity paradoxes. Notably not used with Wesley Dodds AKA The Sandman.
At this point, the remaining survivors actually have it built into their powers. Strangely, back when older versions of the Big Three were part of the team, they aged normally despite having the most leeway to excuse slow aging.
Jessica Drew, also known as the first Spider-Woman, went into cryogenic stasis to speed up the effects of an experimental spider serum her dad injected her with. She went into stasis when she was four, spent at least thirty years in it, and came out looking seventeen.
Ed "Baby Brain" Stargard in Seven Soldiers: Manhattan Guardian. In flashbacks to the Newsboy Army he's a superintelligent baby. In the present he looks like a baby who's got old without maturing.
Cecil in Ghostopolisat first. He looks approximately 10 years old, despite being the ghost of a 60-to-70-year old man.
Neil Nordegraf from the Scott Pilgrim series is generally known as "Young Neil" among his friends due to being the youngest of the gang (at least before Knives Chau showed up). This was played with to such an extent that when Scott finally acknowledged him simply as "Neil" in the final volume, Neil thought it was the best day of his life. Many people will, however, argue that alot of other characters in the series look younger than they make out to be due to the comic's art style, and due to Art Evolution, it becomes less and less obvious. Also, the ESRB even mistook Scott for a child in their parental guide for the video game adaptation.
Monster Girl from Invincible is 29 and looks about 9. Every time she uses her powers she gets younger but her powers get stronger.
Doctor Strange was born in the 1930s but remains 40-something (since that was his age when he met and accepted Death just prior to becoming Sorcerer Supreme). His apprentice and lover Clea appears to be in her twenties, though she's a century or two older than that.
Max from Sam & Max: Freelance Police looks, acts, and sounds (in the game and cartoon adaptations) like a child, but he's actually an adult.
Copycat from DV8 is, like the rest of the Deviants, somewhere in her mid-twenties. However, she's very small and somewhat physically underdeveloped; as her teammate Frostbite puts it, "ain't an ID fake enough to make her look older than thirteen."
In some continuities, Superman. In the Christopher Reeve movies, and sometimes in the comics as well, it's said that the voyage from Krypton to Earth took thousands of years (it was an experimental starship, after all), during which Kal-El aged only slightly—from swaddling infant to toddler, in the movie. Stories taking place in the future generally show Superman's aging to be much slower than that of humans, and that's if he ages at all, with generally the worst of it being him getting some gray hair. One particualr storyline says that he actually shouldn't age at all, and he only does so because he grew up thinking he should.
Jimmy Olsen generally falls under Vague Age, but in one storyline he's captured by a version of Toyman who is an adult-hating child. Toyman says that he can trust Jimmy, since they're both kids—and Jimmy angry kicks him, noting that he's 23.
Supergirl as well, at least in Post Crisis and modern adaptations; she also made the journey from Krypton to Earth, but took longer to reach earth and be released from suspended animation. As such, she looks about 16-18, and is mentally that as well, but is actually a good few years older than Superman himself.
Rangi is the mother of Angus Fangus (a full grown adult, probably in his forties) in Paperinik New Adventures, but she doesn't look much older than him.
Everett Ducklair. He already had two daughters when he first arrived on Earth and lost his memory, but he was mistaken for a late teenager/young adult.