What does 'mature' even mean in this context? Don't mistake "not finished growing/changing" with lacking the ability to make mature, thoughtful decisions if given the opportunity/responsibility.
One of the reasons teenagers act so irresponsibly is because they've grown up with the idea that teenagers are inherently irresponsible and therefore aren't completely accountable for their actions. It becomes self-fulfilling.
Full maturation of the brain - and I do think that indicates being able to reliably make "mature, thoughtful" decisions. I don't think you should deprive teenagers the opportunity to do so in the correct environment, but that's different from assuming they have the same mental faculties as adults.
Personally, I don't feel like I was a mature adult until about 22, and I was never considered an especially immature teen or young adult. I'm 28 now.
>I don't feel like I was a mature adult until about 22
Was that about the time you left college? I think a lot of feeling "mature" is actually having responsibility for yourself. If teenagers had responsibility for themselves, and didn't have the artificial environment of high school to pressure them in the wrong direction, I believe nearly all of them could be fully functioning adults.
I don't think mature brain connections correlates with mature decision making. People become adults when they're finally faced with adult decisions and have to deal with the consequences.
All one needs to make mature decisions is a sense of the consequences of actions and an ability to delay gratification to reach later goals. Both of which is fully within the grasp of teenagers, if they're taught properly growing up.
One of the reasons teenagers act so irresponsibly is because they've grown up with the idea that teenagers are inherently irresponsible and therefore aren't completely accountable for their actions. It becomes self-fulfilling.