A long-term study following 1200 girls in New York, Chicago, and San Francisco found that girls are indeed starting to go through puberty earlier. Back in the 1990s, most girls started developing breasts around age 9 to 9.5. Now, girls are budding an average of four months earlier, which is a significant change in such a short time.
The study proposes a rise in obesity for the cause behind this change, but other scientists have since commented saying that obesity is unlikely the only factor promoting this early change. The amount of dairy and meat in girls' diets, where girls live, and hundreds of added chemicals to foods could also be affecting this shift. Even family stress has changed puberty onset.
Estrogen-mimicking compounds are also a worry. BPAs and estrogen-containing pesticides could set off a flux in hormones for young girls. Nevertheless, much more research is needed in this field. Although stricter regulations in agriculture and the food industry to control hormones in animals and produce certainly wouldn't harm anyone (except maybe Monsanto), because it's difficult to pinpoint where this change is coming from, it's even harder to make any policy changes.