Teens in the 21st century are at a point of extreme mismatch between their biological maturity and their social maturity. The mismatch for girls actually begins in the tween years (9 – 14), while for boys the mismatch may not become a problem until age 16 or so. The mismatch is easier to track with girls because they have a definite biological landmark: menarche.
In the 19th century, girls did not reach menarche until they were about 16.5 years old. By the time they were 18 or 19, they were probably married with their first child on the way. Biological development was in sync with social development. [Girls do not become fully fertile until around 18 months after menarche.]
Beginning in the 20th century, the age at which menarche occurred gradually dropped, while the age at which a teen was considered an adult rose. Now in the first decade of the 21st century, girls reach menarche at an average age of 12.5 years, fully 4 years younger than 150 years ago, but they are generally not considered to be adults for another 8 years or so. And the current average age at first marriage for women is 26 years.
While the average age at menarche is 12.5 years, the range is about 9 – 14 years: those same tween years. It should be no wonder that younger and younger girls are dressing and acting like their older peers. A girl who reaches menarche at age 10 is fertile by age 12. Her hormones are pushing her to mate and reproduce, with unfortunate consequences given how out of sync she is with the currently-accepted age for social maturity. 
The mismatch is just as difficult for boys. In the 19th century, a boy often would have been apprenticed by age 13, learning to do productive work. By the time he completed his apprentice-ship in his late teens, he would have been able to support himself, and maybe also a wife. In the first decades of the 20th century, many young teens left school early to help support their families, despite compulsory schooling laws mandating attendance until age 16. However, the enactment of child labor laws in 1938 in the United States made it difficult for young teens to do productive, paid work.
As with girls, a boy’s age at biological maturity was declining at the same time that the age at which society considered him socially mature was increasing. Gluckman, Beedle, and Hanson (Principles of Evolutionary Medicine) believe “…this mismatch plays a role in teenage depression, acting-out behavior, drug abuse, and suicide.” [p.237] The psychosocial mismatch of the 21st century is the result of unintended consequences of what were otherwise positive changes. Age at biological maturity declined because children are healthier and better fed. Enactment of child labor laws protected children from exploitation. Age at social maturity increased because our society has become more complex, requiring more years of education to achieve success.
Our educational system has changed little since the 19th and early 20th centuries. Yes, students now learn keyboarding instead of typing, but the overall structure and content has changed very little. The result is that biologically mature teens are locked into a structure that treats them as if they were children because society is not yet ready for them to be adults. In fact, the current educational structure may actually encourage social immaturity, thus exacerbating the problem. Biologically mature teens behave like children because we want them to still be children. The result is that due to this mismatch, many teens make unfortunate decisions with life-changing consequences.
