Many traditional peoples set the age of adulthood at puberty. Prior to the Industrial Revolution, that was enough. However, now is a great deal more complicated, and scientists now know (in part due to modern technology for brain imaging) that the frontal lobes do not become fully mature until the mid-20's. The frontal lobes are the part of the brain responsible for complex thought and foresight. That's what's lacking in teenagers . That's why teenagers often get in trouble in very stupid ways. That's why the main causes of death in teens are accidents and suicide.

Yes, a youth at puberty is old enough to take on many of the adult duties in a traditional society; that's why traditional societies carry out coming of age ceremonies at puberty. But traditional societies don't have AR-13's, video games that amount to rehearsal for modern warfare, and radicalizing messages instantly available on a screen in their parents' basement.