Hmm.... I'm over 21, so for me, ... if I could turn 21 again, the first thing I'd do is turn off the computer and go run up and down the stairs. I mean it's just way more fun when you're young and skinny and don't have to worry about anything.
First thing I'd do is wonder where that decade+ of my life had gone to, but then I wouldn't question it, because I'd get to re-live them, hopefully in a different way. If I ended up living the same life over again, I'd have wasted a perfectly good lapse in the space-time continuum.