Dina Fine Maron in Scientific American:
For the past two decades scientists have been trying to unravel a mystery in young girls. Breast development, typical of 11-year-olds a generation ago, is now occurring in more seven-year-olds and, rarely, even in three-year-olds. That precocious development, scientists fear, may increase their risk for cancer or other illnesses later in life. Time has not resolved the puzzle. Nor is there any indication that this trend is slowing. More and more families are finding themselves in the strange position of juggling stuffed animals and puberty talks with their first and second graders.
Obesity appears to be the major factor sending girls into these unchartered waters. The rate of obesity has more than doubled in children over the past 30 years. And whereas only 7 percent of children aged six to 11 were obese in 1980, nearly 18 percent were obese in 2012. The latest studies, however, suggest that weight gain does not explain everything. Family stress and chemical exposures in the environment may also play a role, but the data do not yet paint a very clear picture of their contribution. As for boys, the data are murkier, but one 2012 study did suggest that they, too, may be starting puberty earlier than before—perhaps by as much as six months to two years.