In the Marvel and DC Universes, most superheroes age very slowly (if at all). And even when a hero has "visibly aged" over the years, he may later get "visibly de-aged" on one excuse or another!
In 1938 Superman jump-started the Golden Age of superheroes in "Action Comics #1." In those days, he was a young man, presumably twentysomething years old, just starting a journalistic career as mild-mannered reporter Clark Kent. Today, 69 years later, the latest issues of "Action Comics" still feature a Superman who has no gray hairs, no wrinkles, looks like he could still be in his twenties (if you assume Kryptonians age at the same rate as humans), and is supposedly somewhere in his thirties. If DC is still publishing new Superman comics in another 69 years, I don't expect any of those details will have changed in any significant and permanent fashion by 2076.
Much the same applies to Batman, who debuted in 1939. And his protege Dick Grayson, who debuted in 1940, with the "modern version" of Dick still being no more than "twentysomething" years old today. I will be frankly astounded if Dick Grayson, meaning the "mainstream continuity" version (not an Elseworlds or alternate future timeline's version or whatever) is ever clearly stated to turn "thirty" in any story set in "regular continuity." (And if it did happen, I would expect it to quickly be retconned as soon as the editor who approved the idea was replaced by another one.)
After all the trouble that DC (and other related companies, such as Warner Bros.) have taken to firmly implant in the general consciousness the idea that "Superman is the Last Son of Krypton, aka Clark Kent, mild-mannered reporter at the Daily Planet, and his girlfriend (or wife) is Lois Lane, and so forth," and the idea that "Batman is Bruce Wayne, richest and most eligible bachelor in Gotham, who was scarred for life by witnessing the murders of his parents as a child," and so forth, the chances of their allowing either of those characters to ever get visibly middle-aged, maybe even retire and be replaced by a grown child or other successor as a permanent thing, are right up there with my chances of winning the election for President of the United States next year.
So Superman and Batman will never be more than "thirtysomething." And since they are supposed to live in one coherent universe which they share with their contemporary superheroes, many of whom are roughly the same ages as Supes and Bats, DC appears to feel that if Superman and Batman are going to be perpetually "thirtysomething," then most of their fellow members of the Silver Age and Bronze Age JLA should be locked into the same age range, give or take a few years. (Zatanna may still be in her late twenties for all I can tell; on the other hand, various stories have hinted if not stated that Hal Jordan and Ollie Queen may be past 40 by now -- but in all of those cases, the differences from the ages of Clark and Bruce are probably only a handful of years, rather than a decade or two.)
Marvel has not yet had to jump through as many hoops as DC in this regard, because most of their big-name superhero characters were only created in the 1960s or later -- with a bare handful of conspicuous exceptions, including Captain America, the original Human Torch, and the Sub-Mariner, and the Tricks that have been used on them will be mentioned below! It helps a lot that such characters as Spider-Man, the Fantastic Four, and Daredevil do not have backstories that are strongly tied to specific historical events of the 1930s and 1940s (such as the Great Depression and the Second World War.) But Marvel follows DC's lead by not allowing their heroes to "age in realtime," either. Otherwise the characters who were Professor X's teenaged students in the mid-1960s would all be at least in their late fifties by now . . . unless, of course, some other Trick on my list had been applied to the problem somewhere along the line! (Which it probably would have been! :) )
Here's the list of approaches that I've seen Marvel and/or DC use on various characters in order to keep those corporate assets young and fit for as long as possible:
The 12 Tricks
01. The Ongoing Sliding Timescale Retcon
02. Different species
04. Natural Side-Effect of the Powers
05. Fountain of Youth
06. Replaced Behind the Mask
07. Starting Age Gets Retconned
08. Suspended Animation
09. Time Travel
10. Changing Bodies
01. The Ongoing Sliding Timescale Retcon
"We heroes just don't age as fast as you poor readers! That story you read 20 years ago happened maybe four or five years ago from my perspective!"
In the Marvel and DC Universes, this is the most common approach. So common that longtime fans have learned to just take it for granted without consciously worrying about it most of the time. The basic idea is that time (usually) passes much more slowly within the pages of a monthly superhero title than it does for the fans who are waiting a month at a time for the next installment.
As one classic example of the constant use of the Ongoing Sliding Timescale Retcon, Peter Parker got bitten by that radioactive spider in "Amazing Fantasy #15," originally published in 1962. Today, in 2007, it's been 45 years since that story was published. But you'd better believe it hasn't been nearly that long from Peter's perspective! He doesn't walk around telling his friends and relatives that he got bitten in 1962; he just got bitten "several years ago." (Estimates by fans and writers seem to range from 10-15 years ago, near as I can tell.)
Note: Some people feel that even "10 or 15 years ago" is far too many years, with the result that -- according to rumors I've heard -- the "Ultimate Spider-Man" title is intended to perpetually feature a Peter Parker who, as far as Brian Bendis is concerned, should be and hopefully always will be in high school no matter how long that title endures. An extreme case of being bound and determined to keep a hero (one version of him, anyway) as young as possible forever and ever in order to avoid the problems that have inspired the various Tricks on this list! :)
On a similar note: Marvel and DC often find it useful to work hard to avoid telling us exactly how old a particular hero is at any given time, or how many years, months, and days it's been exactly since a particular old story, so that we fans can only make "estimates" as to just how much slower his aging occurs as a result of the implied Ongoing Sliding Timescale Retcon. For instance, I've never seen Alfred Pennyworth, Tim Drake, and Dick Grayson handing Bruce Wayne a birthday cake that says "Happy 35th" on it -- nor any other scene in "modern Batman continuity" that would constitute a simple and straightforward statement of his exact age! :)
Note: Honesty compels me to mention that while the Ongoing Sliding Timescale Recon is usually presumed to be happening by default in most Marvel and DC superhero titles, with time passing very very slowly for the featured characters, there certainly have been obvious exceptions to that rule in particular cases. For instance! Around 1999-2002, Batman and the supporting cast in his titles were allegedly meant to be aging "in realtime." Dialogue in some of the comics was written to support the idea that something that happened twelve issues earlier had happened "a year ago," or whatever. By the end of 2002, this entire concept appears to have been quietly swept under the rug and never mentioned again! Probably just as well, especially since the vast majority of Batman's contemporary heroes in the DCU (such as his JLA teammates) definitely were not presented as aging another year every time another 12 issues had gone past! I've never understood the logic behind that experiment . . .
02. Different species
"Yep, it's been a heck of a long time since I started the superhero schtick . . . from your point of view as a regular human. But from my point of view, what's a few decades one way or the other? I'm still in my prime!"
Some writers have played around with the idea that as long as Superman stays within range of a yellow sun most of the time, he will not age at all -- or will age much, much slower than any normal person -- over the centuries. After all, he's not even remotely human; so who knows what his Kryptonian metabolism might be capable of doing, under the right circumstances? However, such ideas are usually explored in Elseworlds projects and the like, because it is vitally important to DC to keep Superman's supporting cast from aging too much (Lois Lane must always be about the same age as Clark Kent, Jimmy Olsen must always be a brash young journalist, Perry White must never die or retire; etc.) the Superman stories in the "regular continuity" use other excuses for keeping Superman (one version or another) no more than "thirtysomething" years old at any given time, instead of just having "modern Superman" be the exact same guy who debuted in 1938.
On a similar note: Namor the Sub-Mariner was a young adult in the World War II era, and today he still appears to be in peak condition. No gray hairs, no wrinkles, no heart trouble, no stiffness of the joints. This is explained away by his being half-Atlantean on his mother's side. Atlanteans, we are told, live significantly longer than regular homo sapiens, and Namor definitely takes after that side of the family where his aging is concerned.