ADOLESCENCE WAS BORN a little more than a century ago, in Worcester, Massachusetts. The term has a long history — “adolescens” signified a young man or woman in Latin, and the word first appeared in English in the fifteenth century — but the notion that adolescence is a stage of life all to its own is more recent. The father of the idea was Granville Stanley Hall, a scion of the Mayflower, an amateur magician, and the first man to receive a PhD in psychology in the United States.

In 1904, Hall published Adolescence: Its Psychology and Its Relation to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education. The two volumes ran to more than a thousand pages, and contained more than a small amount of crank. Hall argued that each of us reenacts the stages of human evolution, that primitive people are arrested in childhood, and that women are deficient because they have higher rates of suicide. One contemporary critic wrote, “Chock full of errors, masturbation and Jesus. He is a mad man.”

As Jill Lepore, the Harvard historian and New Yorker staff writer, explains in her latest book, The Mansion of Happiness: A History of Life and Death, Hall’s book nevertheless struck a chord. He placed a great deal of hope in adolescents (Anglo-Saxon ones, at least), and pictured their plastic nature as the crucible of future generations. “Despite our lessening fecundity, our over-schooling, ‘city-fication,’ and spoiling, the affectations we instill and the repressions we practice,” Hall wrote, “they are still the light and hope of the world.” Among well-to-do parents, many of whom shared Hall’s anxieties about shrinking birth rates, urban development and delinquency (thanks to compulsory schooling and child-labor laws, more teenagers than ever before remained dependent on their parents), Hall’s notion that Johnny’s troubles were due to the recapitulation of a primitive state, not to a wicked streak, was welcome. As the New York Times wrote, “Many a puzzled and despairing parent will be glad to learn from this volume that the reason why ‘that boy is so bad’ is not necessarily because he has started a downward road to wickedness and sin. Probably it is only because he has reached the age when it is necessary for him to live through the cave-man epoch of the race.” Evidently, the racialist vein in Hall’s thinking didn’t pose much of a problem to critics. The book sold over twenty-five thousand copies. Adolescence was here to stay.

Aristotle gave us youth, maturity and old age, medievalists threw in childhood, and we’ve since included adolescence: the ages of man have multiplied from three to five. We may soon add another. Two years ago, the New York Times Magazine ran a cover story on “emerging adulthood,” a phase Jeffrey Arnett, a Professor of Psychology at Clark University, where Hall once taught, wants to recognize. Two decades ago, at the University of Missouri, Arnett was studying people of college-age. He asked them questions like “Do you feel you have reached adulthood?” Their responses — one imagines a slow, Lebowski-like “Nah” — persuaded him that the prevailing model of adult development needed to be revised. As Robin Marantz Henig, the reporter for the Times Magazine noted, most psychologists divided adulthood into three parts. Young adulthood from age 20 to age 45, mi...