Tough question honestly (I'm 35 with 3 kids). There's the obvious like banged more chicks, invested more wisely, hand more fun in college (or less fun for some of us).
But really, I don't think I would change a thing because I wouldn't have what I have now if I did. If I went to med school maybe I never marry my wife, maybe I never have my 3 kids etc etc.
Damnit, didn't mean to give a serious answer. I would have screwed more chicks before I got married!