Despite how it may have seemed when you came back from summer vacation to start eighth grade and every boy's voice had changed at once, the age of human adolescence isn't set in stone. A German demographic researcher says that the age of sexual maturity in males has been steadily decreasing since the mid-eighteenth century. And he reached this conclusion based on the rate at which young men get themselves killed.
Human males have an unfortunate tendency to die in early adulthood. Recklessness and violence aren't just traits of young males in the modern era. Even in earlier centuries, when young men had to satisfy their adrenaline cravings with buggy racing, dueling, or hanging around smallpox patients, they were more likely to die than women of the same age.
This leads to a phenomenon called the "accident hump." Both sexes have a relatively high risk of dying as an infant, but that risk plummets throughout childhood. Mortality bottoms out in older kids, when the threat of childhood disease recedes and adult ailments are still far in the future. As adolescent girls move into adulthood, their risk of death slowly and steadily climbs. Each adult year that you're alive has a greater chance of being your last. But for males, the mortality rate takes a sudden leap above females in early adulthood. This is the accident hump. As men get a little older and leave behind their risky or violent behaviors, their risk of death smooths out again to match the steady climb of female mortality.
Joshua Goldstein, at the Max Planck Institute for Demographic Research, says that the accident hump has been shown to coincide with sexual maturity, the time when male bodies are cranking out their highest levels of testosterone. Other studies have shown that female sexual maturity, measured by the age when menstruation starts, has come earlier and earlier over the past few centuries. As humans have developed better medicine and nutrition, our bodies have been able to grow bigger and mature sooner. Has sexual maturity also been arriving earlier for males?
To find out, Goldstein looked at death data from several European countries with thorough records. The dataset began in the mid-1700s. In each decade, he determined the age at which the accident hump peaked; this would roughly represent the average age of male sexual maturity.
He found that throughout the nineteenth and twentieth centuries, the accident hump came about 0.2 years earlier in each decade. In the late 1800s, young European men were dying most often around age 21. In 2000, that age was closer to 18.
So at the same time that humans were getting bigger and healthier, the peak of males' dangerous behavior was getting earlier. Has the onset of male adolescence also grown earlier? Goldstein says there's other evidence beside mortality rates. A study of boys' choirs found that in a choir lead by J. S. Bach in mid-eighteenth-century Germany, boys' voices didn't change until they were 18 years old, on average. But in twentieth-century London choirs, boys' voices changed much earlier, around age 13.
Earlier sexual maturity might mean teenagers are driven to try risky behaviors (and girls are able to get pregnant) at increasingly younger ages. Goldstein wonders how brain development, which continues into early adulthood, compares to sexual development. If the timing of brain maturation has stayed the same while physical maturation has grown earlier, does this make for an increasing gap between physical and mental adulthood? Goldstein also points out that in the past half-century or so, cultures around the world have seen the milestones of adulthood--marriage, financial independence, parenthood--pushed to later in life. Essentially, though most of us would have cringed to hear it in seventh grade, we may all be living out a longer adolescence.
Goldstein, J. (2011). A Secular Trend toward Earlier Male Sexual Maturity: Evidence from Shifting Ages of Male Young Adult Mortality PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0014826