Heh. Having actually lived during the '80s, we had the recently late Raquel Welch still a sex symbol in her forties. Paul Newman and Robert Redford were still considered highly attractive to the ladies, while Cary Grant in his eighties still was still considered handsome. Elizabeth Taylor by the mid-'80s regained her glamorous image once again, while Joan Collins was still still considered sexy. There are other examples, too, so this alternate Earth you speak of is different from the one I lived on.Why wouldn't Robert Pattinson be a valid example? He is literally how DC/WB decided to portray Batman. The point I'm arguing is not that if a 50 year would be an effective running back in the real world (I don't even know what a running back is), the point I'm disagreeing with you is that "DC would never age the Trinity above age XX because of their iconic status". And for this, the underlying logic of professional high impact sports is not that important: yes, in American football specifically, or several kinds of martial arts, there's only so much abuse one can take before having to call it quits because it adds up. Batman doesn't have this concern, he can take infinite abuse and be healed next month. The very tiny amount of realism we have to take into account for is more on the line of "How strong/fast/good looking can a 40 or fifty year old be. How interesting a life can they lead?". And for those aspects, the social perception has changed radically, for a variety of reasons. Now, in the 80s, let's say, "40" evoked the image of a guy with a beer belly, receding hairline and screaming at kids to get off their yard. It was important for DC to keep their main properties under this hypothetical age because of this. Now, that's not so true anymore, and said fact may as well be reflected in comics as it already is in movies or animation.
But here's how a middle-aged Batman could thrive: he uses the Lazurus Pit judiciously or some other medical wonder to keep him still young, vibrant, and at his peak potential. Not that he couldn't be the latter, regardless, since we're talking about suspending-your-disbelief comics, but at least it would make a lot more sense with some sort of explanation. Though I suspect those wanting to push Bruce into middle age really don't want him to be at his best so a legacy can get the predominant Bat exposure, so that would cause a problem.