Jessa Gamble in The Atlantic:
In a time when college graduates return to live under their parents’ roofs and top careers require years of internships and graduate degrees, the age of adulthood is receding, practically into the 30s. Adolescence, loosely defined as the period between puberty and financial independence, now lasts about 15 years, twice as long as it did in the 1950s. Part of this is due to the declining age of puberty in both males and females, but most of that extension appears in the 20s, when an increasing number of young people are still dependent on their parents. There is some concern that all of this dependence could lead to a lasting immaturity and failure to take on responsibility.
But according to developmental researchers, there is one lasting gift that extended adolescence can bestow, and it resides in the brain. “Neurobiological capital” is built through a protracted period of learning capacity in the brain, and it is a privilege that comes to those lucky enough to enjoy intellectually stimulating environments in late adolescence. Far from a contributor to emotional immaturity, the trend toward an adolescence that extends into the mid-20s is an opportunity to create a lifelong brain-based advantage. 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.