My sons and I have been talking about this. Then I saw a study on it on Reuter's Health.
My boys and I talked about being able to drive–16 here. 17 in NY. 15 in Louisiana.
We talked about being able to leave home– also 16.
We talked about voting. 18.
We talked about legally drinking. 21
We talked about why those ages are different. (If I can get killed in a conscripted war, why can't I drink? That was the question which brought the drinking age down to 18 when I was young. Getting rid of the draft brought it back up.)
I have always told the boys that they're grown up when they take responsibility for themselves and their actions. (I know some non-grown up 30somethings.)
This study says that people think getting an education is the first clue to adulthood. (You can tell these representative sample folks were at least middle class.) I thought it ought to be living away from your parents and supporting yourself.