I'm approaching 29 and I feel perhaps a little different than I did at 18, but not much. It didn't start to kick in at all until my mid-twenties. Working full time and seeing my friends less because they moved further away and our schedules don't match etc. I'm just starting to think of myself as a grown-up now that 30 is in sight and I can't say I really approve. Being a full decade removed from my teenage self is a surreal concept.
I tend to think of anyone around 23 or younger as a kid now. They look so young. Anyone younger than 20 is just obnoxious. None of this is probably helped by the fact that I spend most of my time with people older than me.
“Today a young man on acid realized that all matter is merely energy condensed to a slow vibration, that we are all one consciousness experiencing itself subjectively, there is no such thing as death, life is only a dream, and we are the imagination of ourselves.” – Bill Hicks