well, i'm still 43 (or was i 42 when i posted last...?). anyway, all sorts of exciting things happen once you get older, and have lived a bit, and have learned more (well, in theory - there's no hope for some people), and learned how to suffer the slings and arrows etc., and can cope better with the vicissitudes of life.
apart from anything else, you can always chuckle at younger folk doing the same things you did, and getting all excited about it, or annoyred, etc., and be reeeeeeallllly smug about it. if that's what gets you going, of course.
i wouldn't be 20 again if you paid me. unless it was a) an inordinate amount of money, or b) with everything i've learned since i was 20 the first time.