I'd say 40's, old enough to know better, to not care so much about the judgement of others , to be somewhat financially stable ,and young enough to go out and enjoy that knowledge with an intact sense of adventure!! Fun!:-)
30's, You've had enough experience to have better judgement on things. If you've been good enough to live that long and not be in jail you should be in a really good spot to ENJOY life in a way that you're young enough to be at your best physical condition and not too old to like the point that your body isn't moving like it used to.
To me, it's not the age group what makes it fun... What's fun is making everyday of your life fun and worth living and you will never regret a thing(but still obviously be considerate of others and don't take advantage of them)...then when you get older, you will realize your whole life was fun...then you have so much stories to tell your children, your grand children and your grand-grand children :) just how I see life should be spent!
its different for everyone even though most say 20's never know some may have kept care of there bodies(like still look like there 20) and struggled in there 20s(job, home, law, ect) and have better 30s/40s.
I'm 100% with Wittepier on this one. The forties are the best.
As a child and in your teens you're still learning how to put it all together. In your 20s and 30s you actually have to do just that. But in your 40s you should be pretty well established, your kids will be old enough to look after themselves if you decide to do something as a couple. And you're still young enough, and can be fit enough, to do anything.
Daizy (that's Mrs Dodgy) loved her 40s. Until then she'd been my wife or the kids' mother but in her 40s she started to get fit, took up triathlon, won an age group State championship, went back to school and matriculated. For her, this was the decade that allowed her to be an individual. She has never looked back.