As it happens, I'd be content to cast an actor who can pass for younger (in Begins Bale was 31 playing Bruce Wayne at varied ages between his teens and 30), but I'd have to reluctantly agree that having a Batman aged 30 at the start of his career was perhaps a questionable decision - bearing in mind that that's an age where many athletes and sportspeople are viewed as having their best days behind them.
That comparison really makes very little sense. Sports and acting are two completely different things. The requirement for an actor is to act, and secondary, to look the part (which makeup and camerawork can help a lot with), rather than, say, to run really fast across the court to catch the ball coming from the other side of the net. By your logic, they should be casting top athletes in all the roles that require any action scenes (despite the fact that most actors don't do their own stunts... and it's not like stuntpeople are that young, either) and the top actors in Hollywood blockbusters should be those just out of their teens and should quit at 40 since they obviously have nothing more to offer, not being able to run that fast as they used to.