Humans have achieved 85% of adult brain volume by age two, but the brain is not fully "wired" until the mid-twenties, long after sexual and other physical maturity. This has important consequences.
Think about this— adolescence and early adulthood are the times when someone is most likely to kill, be killed, leave home forever, invent an art form, help overthrow a dictator, ethnically cleanse a village, devote themselves to the needy, become addicted, marry outside their group, transform physics, have hideous fashion taste, break their neck recreationally, commit their life to God, mug an old lady, or be convinced that all of history has converged to make this moment the most consequential, the most fraught with peril and promise, the most demanding that they get involved and make a difference. In other words, it’s the time of life of maximal risk taking, novelty seeking, and affiliation with peers. All because of that immature frontal cortex.
Sapolsky, Robert M.. Behave: The Biology of Humans at Our Best and Worst (pp. 200-201). Penguin Publishing Group. Kindle Edition.The last part of the brain to mature is the evolutionarily most recent - the dorsolateral prefrontal cortex (dlPFC), responsible for much of our higher level thinking. It's the part of the brain where you are supposed to "think-twice" about punching out that cop or making that cutting remark to the boss.
Another Sapolsky tidbit concerns the fact that in early adolescence the brain is packing on gray matter (neurons and connections) but later adolescence is devoted to pruning and optimizing that neural network. It seems that tracing development with neural MRI and IQ tests shows that:
The longer the period of packing on gray-matter cortical thickness in early adolescence before the pruning started, the higher the adult IQ.
Sapolsky, Robert M.. Behave: The Biology of Humans at Our Best and Worst (p. 203). Penguin Publishing Group. Kindle Edition.And:
In the foothills of the Sierras are California Caverns, a cave system that leads, after an initial narrow, twisting 30-foot descent down a hole, to an abrupt 180-foot drop (now navigable by rappelling). The Park Service has found skeletons at the bottom dating back centuries, explorers who took one step too far in the gloom. And the skeletons are always those of adolescents.
Sapolsky, Robert M.. Behave: The Biology of Humans at Our Best and Worst (p. 207). Penguin Publishing Group. Kindle Edition.