I partied pretty hard from my mid teenage years through my early 20s. Had a lot of fun for a while, but I eventually got sick of being f'd up all the time. My academics suffered greatly from late high school to about my junior year of college; I almost failed out at a couple points. I started seeing my friends making decisions that would irrevocably messed up their lives and a handful of them died super young. Eventually I got to the point where I realized my lifestyle was unsustainable that I had to make a "now or never" sort of decision to clean up my act, as it would just get harder and harder as time went on. I started focusing more on school, started eating right, kicked some nasty habits, and started running.
Looking back, I'm happy I got all of that craziness out of my system early. I'm in my early 30s now and know some "late bloomers" who party like they just turned 21, which really puts me off. Don't get me wrong, I still like to hang out and throw back a few drinks with friends and I don't really regret my wilder years, but I'm much happier with my current situation. Also, I think it's important for kids to party, make mistakes, and have fun, you just have to make sure it doesn't get out of your control.