(Edit to add): It's been known for a while that physiological maturity of the brain occurs after age 20:
"Adulthood: What the Brain Says About Maturity" http://www.nytimes.com/roomfordebate/2012/05/28/do-we-need-t...
Neuroscientists now know that brain maturation continues far later into development than had been believed previously. Significant changes in brain anatomy and activity are still taking place during young adulthood, especially in prefrontal regions that are important for planning ahead, anticipating the future consequences of one’s decisions, controlling impulses, and comparing risk and reward. Indeed, some brain regions and systems do not reach full maturity until the early or mid-20s.
"The Adolescent Brain -- Why Teenagers Think and Act Differently" http://www.edinformatics.com/news/teenage_brains.htm
A recent study by Lebel and Beaulieu (see below) reinforce the above findings that the human brain doesn't stop developing at adolescence, but continues well into our 20s.
My "if it develops at all" comment has more to do with theories of cognitive development -- higher levels of functioning are fairly rare. Jean Piaget's work is recommended:
https://en.wikipedia.org/wiki/Maturity_(psychological)
https://en.wikipedia.org/wiki/Jean_Piaget%27s_theory_of_Cogn...
My speculation that full intellectual maturity lags physiological mature is my own, I've no references on that.
> Remembering that I’ll be dead soon is the most important tool I’ve ever encountered to help me make the big choices in life. Because almost everything—all external expectations, all pride, all fear of embarrassment or failure—these things just fall away in the face of death, leaving what is truly important
The Dalai Lama too:
> Half of our lives we spend asleep. The first ten years we are merely children, and after twenty we begin to grow old. Meanwhile, our time is taken up with suffering, anxiety, fighting, sickness, and so forth, all of which limit our ability to practice