Most girls don't start developing breasts until around the fifth grade. But as many pediatricians, parents and teachers will tell you, this first sign of puberty seems to be occurring more often among six-year-olds and seven-year-olds than ever before. (Boys, as far as anyone can tell, are still growing up at their usual, slower pace.) Nobody can explain this speedup. Some even question whether it's real or anything abnormal. But if you suspect your first- or second-grader is blossoming too early, some basic information may help you sort through the confusion.
Researchers don't know exactly how a child's body decides to...