I've never caught a foul ball before, but at this point I'm so old that it would be weird to keep it even if I do catch one. I'd have to give the ball to a little kid.
Little kids can do whatever they want though. They can keep the ball and no one would think twice about it. Or, on the other hand, they could also get their game on by giving the ball to the pretty girl sitting behind them, like this kid:
Smooth, kid. Very smooth.