Superman and Batman are probably both in their late 30’s. Shortly after he was killed by Doomsday, Clark Kent was listed among the missing, and his bio listed him as 34 years old. That was roughly 15 years ago real time, which would translate to roughly four or five years DC time. In addition Clark graduated with Pete Ross, who was elected vice-president a couple of years ago. This makes him, and consequently Clark, at a bare minimum, 37-38.
Batman graduated from college and spent several years traveling the East training with the great martial arts masters. Batman became Dick Grayson’s guardian when he was 12 or 13, and this was at least three or four years into his Batman career (Dick doesn’t appear in Year One or Year Two). Dick has graduated from college and has been on his own as Nightwing for a couple of years.
Assume that Bruce graduates at 22, finishes his martial arts training and begins his career as Batman at 25. He becomes Dick’s guardian at around 27, early in year three, with Dick about 13. Dick graduates at 22 and becomes Nightwing, which he’s been for at least a couple of years, so add another 11 years to Bruce’s age and you get 38.
Batman and Superman both came onto the scene at about the same time, so assuming they’re both about the same age, I’d put both at just under forty.
There are other clues. Batman and Superman were both in the first incarnation of the Justice League (though not charter members), along with the Barry Allen Flash. Wally West was about 9 at that time (the age when he first got his powers) and is now in his early 20’s, call that 12 or 13 years. Asssuming an age of about 25 for Bats and Supes, at that time, that fits with the idea that they’re just under 40.
However, if you look at Black Canary and Hawkgirl, their timelines seem to indicate that the first Justice league was closer to 20 years ago. The current Black Canary is in her late teens/early 20’s, and is the daughter of the original. The current Hawkgirl is also in her late teens, and is the reincarnation of the Silver Age Hawkgirl. This would imply that it’s been 20 years since the originals were in the first Justice League. Using those clues, Batman and Superman would be in their early 40’s.
One last clue is their hair. Dark haired men in comics who are in their forties or later have gray hair at their temples. This means that Superman and Batman cannot yet be forty.
So my official estimate is 38, give or take a year.