People have a habit of dividing life into segments. The psychologist Jean Piaget argued that children go through four stages of cognitive development. Biologists describe turning points in the aging process as though they’re cliffs from which we’re doomed to fall; at roughly forty-four and sixty years of age, for example, distinct waves of molecular changes seem to increase our risk of many diseases. “Passages: Predictable Crises of Adult Life,” a 1976 best-seller by Gail Sheehy, warned of restlessness and infidelity among women starting at age thirty-five—incidentally, the age when I married. Last summer, I woke up to the good news that, according to economists, the midlife crisis is vanishing. Oh, wait: apparently young people are now unhappy enough that entering middle age seems rosy by comparison.