Teen Brains: Why Are They the Way They Are?

Teenaged brains are different from adult brains.  The neurons in the frontal cortex of an adolescent brain are among the last to become fully myelinated.  Myelin acts like insulation on an electric cord, making nerve impulses pass along faster and more effieciently.  So those adolescent frontal cortex neurons are less useful than they will be in a few years.  
    Alas, the frontal cortex is the site where conscious judgment takes place.  So teens are somewhat lacking in judgment.  They don’t assess situations very well, and they make poor, sometimes risky choices.
    How could Nature have allowed this situation to evolve?  Why hasn’t natural selection long since weeded out such un-fitness?  All we can do is guess.  But to do such guessing, it helps to remember that for tens of thousands of years, the life and social organization of our species has been quite different from present times in developed countries.  
    People lived in clans or tribes, and elders were in charge.  (This is still the case in many parts of the world.)  So poor judgment in adolescents may not have been a problem.  In fact, an immature frontal cortex may have helped.  One way the brain matures is by pruning:  Connections among immature neurons that aren’t being used, die away; whereas those being used a lot become strengthened.  So the influence of elders on the adolescent may have become permanent in this way.
    Another possible advantage of an immature frontal cortex in teenagers may be courage.  These young people must now leave home and start lives of their own.  It’s probably no accident that we humans have elaborate rituals and ceremonies around becoming adult and getting married.  In any culture, whether teens leave home physically or merely psychologically, they must be brave enough to let go of parental protection, to become adults among adults, to marry and take on the responsibility of raising their own children.  Even in societies where very young women are married off to older men, courage is a requirement.
    There are probably lots of other very good reasons to view teen brains as highly fit for their purpose.  Let me know your ideas.
    Meanwhile, I’m not giving up on Nature!