You chose to ignore all of the obvious signs of adulthood. Instead you want to cherry-pick a few insignificant changes in the brain and say "these are the only things that matter for maturity"
Sorry. That's just very stupid.
It would be equally valid to claim that pubic hair, or menstruation, or any other adult trait is the only one that matters.
You think that there's a difference between 26 year olds and 16 year olds. Well, yes, there is. But many 26 year olds can pass for 16 and vice versa. Because they are both adults. 16 year olds cannot pass for 6 year olds or vice versa. 6 year olds are children.
After puberty humans are adults. This is not difficult. Desperately searching for neuological data so you can claim 17 year olds are children is dumb. Stop doing it. Nobody benefits from you supporting bigotry. Stop it.
Odd how our bodies reach adult height and brains adult size during puberty :-)
You have cherrypicked your sources. All of neurology points to very fact that brains develop very quickly ages 9-13 and then very slow change from that point on.
but that data doesn't support your bigotry, so you ignore it.