There really are not FACTS to prove the claim that kids are physically maturing faster... Yes, it could be true. The hormones added to things are having an effect. But also look into genetics. ANd wonder why BOYS are not developing sooner also... How many 9/10 yr old boys have the deep voices of a 13 /14 yr old boy? lol Not many at all... Yet girls maturing "faster" is the only example people give for the hormonal effect.
I grew up on an ORGANIC dairy farm. We raised ALL our own cows (which we used for milk, meat, and cheese) sheep (meat), chickens(meat), and we had a huge garden every year. My family did eat store bought junk food (chips, soda, etc) occasionally... I started my cycle when I was 13... I have a friend I grew up with... Their family purchased their food from us and what we did not have they ate strictly organic. They avoided sugar and fried foods. A much healthier diet than I grew up with... She started her cycle when she was 9 yrs old... Her mom was 10, her 2 sisters were 8 and 11 when they started...
One thing that could also play into it is the higher amount of refined sugars and fats we all have in our diets today. Breasts are fatty... So the more fatty foods you put in the faster (or maybe larger) you will develop them. Also our life styles are more sedentary than any previous generation... Even if eating nothing but fatty foods, if our kids had to walk a couple miles to school daily and help out on the farms doing chores and helping do physical labor our bodies would be much healthier.
I do think that activity plays a part in it. Athletes that train really hard do get to the point that thier bodys cycle stops.
I do not blame ONE thing for changes. Its a dominoe effect. One thing leads to another.
Do what you can to avoid the hormones if that is what your worried about, buy milk that does not have it in, hormone free meat, if you can afford it... But also understand that it may not stop your daughter from starting her cycle earlier than you did. :-)
ps. my daughter is 8... no cycle... My niece is 12 and yes, she has some "buds" but not cycling yet...