Obesity is also a factor - chubby girls tend to start earlier than thin girls do. In fact athletic, thin girls are often the last.
“I read something a while back that said soy in the diet has a major effect.”
Non-fermented soy is a major player here. Soy is estrogenic. Soy was never meant to be food for humans or for animals. The far east uses much lower amounts of soy in their diet, and it is mostly fermented soy.
Soy milk is a major source of the problem in children. Soy milk should never be consumed by anyone. It induces prematuree puberty in young girls, causes delayed puberty in young boys, and soy is cancer inducing because it is estrogenic.
I believe this. Many in the scientific community also believe it. There have been studies, papers written. Google it if you have doubts. You will find info supporting the hazards of soy, those saying it is poppy-cock. I choose to believe soy is a serious hazard.