Great thread.
I went through a lot as a kid (20 surgeries in 16 years, 18 of which before I was 8), and I think that made me feel more mature than others my age... but I never felt "grown up" until I bought my house at 24. Turning 30 last year pretty much cemented it for me though. Realizing that I'm no longer in my 20's, and that the stuff I started way back when (buying the house, starting my retirement fund) will actually pay off, and sooner than I think.
Despite all that I still do "kid stuff" occasionally... I still watch cartoons, and one of my favorite places to go on vacation is Disneyworld.
Youth is all in your mind, age has nothing to do with it.
Dave