Choirmasters’ records show that whereas choristers’ voices broke around age 18 in the mid-1700s, that age declined to 13 in 1960, and voices now break on average at age 10-and-a-half. Meanwhile, the age of first menstruation in girls has been declining by more than three months per decade. Much of that change comes down to improved nutrition, but in recent years the age drop has become a health concern. Marriage and financial security, on the other side of adolescence, now arrive close to age 30, in contrast to the early-20s marriages of the 1950s. In combination, those changes make for a more dominant life stage between childhood and adulthood.

Biologically, adolescence serves to prepare the brain for independence, and it represents the last surge of plasticity, when the brain is far more open to change than it was in middle childhood. The child-development expert Stuart Shanker says this pushes the teenager out of the house, replacing the overly comfortable embrace of the parent with the riskier world of peers and autonomy.

“Here, nature has spent eight or nine years getting the brain into a stable state in terms of higher functions, and then it’s all suddenly thrown back into chaos,” says Shanker. “We see changes in their reward system, which drives them into peer activities.”

The central task of adolescence is to develop the ability to self-regulate—to manage stress so that it does not interfere with the ability to follow through on plans and delay gratification. In particular, connectivity develops between the prefrontal cortex, crucial for executive functions like attention and decision-making, and the limbic system that drives emotion and reward.

Adults vary widely in the degree of success they achieve with this type of impulse control, and it has a large bearing on their life outcomes in work and relationships. Depression, obesity, and substance abuse are all exacerbated by self-regulation deficiencies, and adolescents who miss the opportunity to develop these capacities disproportionately run afoul of the law. For a teenager, potential rewards are almost irresistibly appealing, and the attendant risks seem worth it. The tendency to bet on a low-probability outcome is highest at age 16, and arrests peak at 18.

It’s not a matter of logic. In fact, a 15-year-old can reason and problem-solve as well as an adult in many areas of calculation, and they can accurately identify the risks of drug taking, unsafe sex, and crime. It’s when incentives, fatigue, social pressures, or stress come into play that an adolescent becomes more susceptible to irrational thinking. Last to develop are the circuits that deal with navigating other people’s needs and emotions, pushing past failure without losing determination, and connecting immediate actions to long-term consequences. With a longer adolescence, some people have an extra chance to strengthen these human capacities.