I’m 24 and I would not claim to be “grown up”. Yes, I passed the 18 and 21 year marks, but determining the exact point at which you feel part of “the adults” you once looked up to, appears difficult.
Maybe the point at which you start recognizing the experiences your parents tell you of their past. It’s a weird feeling when they tell stories similar to what you heard from friends or lived yourself.
It’s a point where you may realize your own experience in life. The only thing separating you from your parents are the extra years, but the fundamental something that distinguished their world from yours seems dissolved. Your expriences now happen in a similar world with similar terms and conditions, the same chapter in life, adulthood.
Just a very sentimental thought ;)