We encouraged both kids to leave home by the time they were 21. We told them that if things didn't work out, they could come back. Neither of them has done so, I'm proud to say.
I left home soon after graduating from High School. I went back for a short time (like maybe three months) just before I joined the Navy at 21. It was hard to make it on my own, but it was harder to put up with the parents and the little brother. So honestly, I took the path of least resistance; it could just as easily have been a decision to stay at home, had the parents and sibling been different.
It takes guts to be independent, but it also takes a certain amount of rebellion. The latter is as negative a quality as the former is positive.
On the other hand, by 21, one should be trying to sustain oneself and take responsibility for oneself without the aid of the parents. The thought of needing parental aid should be repugnant, so repugnant that one would work 2 jobs before resorting to it.
In the case of both of our kids, (now 25 and 23) they won't even let us buy textbooks for their college courses, and they're putting themselves through college while working and paying rent. I'm VERY proud of them both!