The average age is now a little over 9. That's two years younger than past decades.
The lowest is for African-American girls - the median age is 8-point-8 years.
The highest is for Asian girls - 9-point-7 years.
Puberty is also at 9-point-7 years for white girls and 9-point-3 years for Hispanic girls.
However, doctors say body-mass-index--- that's the relationship of height to weight...is the best predictor - the higher a girl's B-M-I, the more likely she is to start puberty early.
And that can lead to health problems later.
"If you finish puberty earlier in life it can put you at risk for some diseases long-term. Things like breast cancer, which is probably a large concern in this group," says pediatrician Dr. Sara Lappe.
Experts don't think obesity is the sole factor behind early puberty - they think chemicals in the environment may also help trigger the hormonal changes.
Doctors date the start of puberty from the beginning of breast development. Menstruation usually begins 1 to 2 years later.