Honestly, it's just a number for many of them.
Clark, Diana, and J'onn are all long lived in many versions and Diana's straight up immortal in some. And a lot of metahumans like Barry are believed to have their lifespans extended due to their powers (just like the surviving JSA members). And for the rest, they're hyper-active physical specimens who spend their whole lives training; it's not out of the question that they could be active long into age ranges where us mere mortals would've settled down for a desk job and a beer gut.
Hell, the vast majority of metahumans could easily have extended lifespans just as a byproduct of their powers. Even someone like Canary, who has a pretty straightforward and simple power, has to have an evolved biology to process and adapt and survive her own abilities. Dinah's skin would have to be tougher to withstand her own sonics (not bulletproof, but definitely thicker than the average dermal layer) her ears would have to process sound differently, etc. Is it hard to believe that her internal organs are also heartier and more durable and would thus add a few extra decades?
And really......I think age is a foolish thing to get hung up on. In the real world, Batman would've died his first week in the cowl. So is it really hard to believe that he's kicking ass at 40? Tom Brady (my man!!!!!) just won his 6th Super Bowl ring at 40, so if he can do that in real life, Bruce can kick people in the face at 45, or 50, or whatever.
It's fiction. It doesnt have to fit reality.