It takes so long to “grow up” today—finish education, find a stable job, get married—that it makes sense to think of it as a new life stage, emerging adulthood, in between adolescence and young adulthood. But why? Four revolutions of the 1960s and ‘70s explain it: the Technology Revolution that changed the economy and led to more education for longer than ever before; the Sexual Revolution that made it possible to have a sex life without marriage; the Women’s Movement that gave women an incentive to postpone marriage to pursue a career; and the Youth Movement, which gave adulthood a bad name. Older adults grumble that it takes “too long” to grow up today, but most people will be better partners and parents if they wait longer, so why not use most of the twenties for adventures you’ll never be able to have again?