The discussion sounds a lot like 'when do you become a man' than it does 'when do you become an adult.'
For instance, most women throughout history were financially dependent on their husband. They would not make independent decisions. They were not solely responsible.
Did that mean they weren't adults?
The 20-30% of women who still become stay at home moms... they can't grow a beard... they are dependent on their husband or parents for money...