Actually that is attributed to diets that are higher in fat than what was being consumed years ago.

A womans body is dependent on fat content in order to enter puberty. When the correct level is reached the body starts the changes. (This is why some gymnasts do not enter puberty until after they stop being gymnasts, there are 20+ year old women who have not gone through puberty yet due to things like this. It also happens with eating disorders quite a bit.)

Due to the diets that are higher in fat content today young girls are entering puberty at a younger age than seen previously, also because one of the major contributors to breast size is caloric intake young women are developing larger breasts more quickly.

In general entering puberty sooner is not a bad thing, the worry is that the diet is doing more than that though. A concern is of course the effects of a diet that contains over 40% caloric content at such a young age. The gene that controls the bodies ability to store fat is triggered at this level of caloric content and leads the body to be more efficient at storing fat. This has the effect of making a person more likely to be over weight and also makes it more difficult for them to lose or maintain a more "normal" body weight based on size and body type. Epigenetics trigger DNA changes most easily during childhood and through puberty, this is why this is such a crucial time to maintain a healthy diet and lifestyle so that you do not end up paying for poor choices for the rest of your life. However more and more we are seeing young people go with quick foods that do not make for healthy choices.

Wow, I did not intend to write so much on that, it just sort of happened. The sad fact is this is sort of cutting the explanation short and leaving a bit out.

Good day.