I can't see them casting someone in their mid to late forties - surely going to be early thirties like Superman.
I was going to say the same thing, but then I remembered Robert Downey, Jr. was already in his 40s when he first played Iron Man. And contrasting the young, learning Superman with a more seasoned Batman has potential. So I agree it's more likely they'll go for early 30s, but I'm not completely ruling out the alternative.
Stark has always been middle aged though. Plus the character doesn't need to be in perfect physical shape, since he drives a high tech armor.
Batman needs to be relatively young, unless they want to go more Batman Beyond old mentor role. But I doubt the WB will want that since it is in their best interests to get another younger actor to start another solo Batman franchise.