For me mid to late 20s is (was) a great age to be. Late teens/early 20s can be exhilarating and tons of fun, but there's still so much to learn about yourself and other people, and some of that can actually be pretty painful to deal with.. there's love (girlfriends), and moving away from the safe cocoon of family and discovering new places/ideas, and finding yourself- if you're not ready for it some things can just come up and hit you like a ton of bricks, at least that was my experience. Early 20's can be rough.
Late 20's, early 30s (I'm 32 now) seems to be an altogether smoother ride, which I'm grateful for. I still don't have kids of my own so I'm probably starting to fall a little behind the average curve of experience here, but that's ok. I'm also back at school which is probably making me feel a little younger than I actually should. Not going to complain though.