well, youth sells better... what a lot of people dont know is that usually the management tells artists to hide balding for example, because it wont sell to todays youth...I think Hollyweird male celebs generally like em young because those girls are more groupie like in their behavior (starstruck) + it makes them feel more attached to their youth that they miss so much.
so yeah they got a hard time getting older and just compensate with dating young women, especially leo (whom i actually think is quite good as an actor, so no bias here)....