I remember when I was really young, there was a T.V. show called "Thirtysomething." I have no idea what is was about, but remember thinking how incredibly old people in their 30's must be. They probably don't have any fun, probably hurt their hips if they attempt any crazy activity and probably all drink prune juice.
Tomorrow (the 26th) I will wake up no longer in my 20's. I feel quite confident I will still act, look and talk the same, but still can't shake the feeling I'm getting old.
Along those lines, I always imagined people who are 30 are "grown up." Yeah.... Other than my hair turning a little more gray, I still feel young. I still kinda look young. And I definitely don't think of myself as all that mature. I purposely say dumb things, make jokes normally made famous by junior high kids, and pretend I could still fit in with the college kids if I only shaved my beard.
I guess "young" is simply an opinion that operates on a sliding scale. It will continue to be at least several years ahead of me until my kids convince me otherwise.