On the one hand, I've felt grown up since I was about three years old. On the other hand, I don't feel grown up now in a lot of ways.
Looking back, I guess I was launched into premature "grown-up-ness" the year I was in an accident that killed my childhood friend, months after a couple of other deaths of young people in my life - with my father dying only months after that. I was 20/21. By the time I really got past all that stuff/tragedy, I was in my mid-twenties and adopting a baby who needed a mother. It wasn't long after that that I was having premature babies and miscarriages, and I found myself with three children and whole lot of responsibility. Some more grown-up worries and issues kicked in as well. I guess the last, tail-end, years of carefree youth were wiped out that one awful year, and by the time I was over it all I was already entrenched in grown-up life - never to be able to regain that same "innocence of youth" that I'd had before.
That grown-up life has been far better than any teen years had ever been, so it's not all bad. It's just that it might have been nicer if I'd had a couple more years of feeling young and carefree before settling into the more meaningful (and whole), adult, life. The good side to that bad year, however, was that I learned how important it is not to let life take too much youthfulness away from us - so all these years later, I'm happy to say I've been very skilled at safeguarding what was left of my "young thinking", and of what was left of a happy young girl after tragedy swept in and stole whatever it was able to steal from her.
If you saw the movie, Forrest Gump, you may recall how Forrest said something like, "I may not be a smart man, but I know how to love." The thing I would say is, "I may not have had a charmed end to my teen years, but I know how not to be too grown-up and old." Being grown-up, I've found, is something we have to do when the situation calls for it. It's never something we have to be when the situation doesn't require it.