Natasha Gardner on girls and early puberty, and what happens after they grow up:

In the United States, menstruation generally arrives when a girl is 12-and-a-half years old; that hasn’t changed significantly in recent decades. What has changed is the onset of early, or “precocious,” puberty, which triggers breast development and pubic hair as early as seven (for boys, the cutoff is nine). Until the late 1990s, researchers thought puberty usually started around age 11. By 1997, the average age bumped down to 9.96 years; today, it may start as early as six-and-a-half years old. Although environmental chemicals, hormone levels in water supplies, and mere evolution have been cited as possible culprits, researchers still don’t know why the shift is happening.