A new study has identifited that many girls will reach puberty before the age of nine - months earlier than a decade previously.
Researchers from Cincinnati Children's Hospital suggest that this is linked to increasing rates of childhood obesity.
Caucasian girls start to develop breasts at about nine and a half while African-American girls now start to develop breasts just before they turn nine, on average.
The researchers studied data from 1,200 American girls who were enrolled in the study between the ages of six and eight.
Prime driver behind early puberty
Study lead Dr Frank Biro wrote that rising obesity rates seem to be a ‘prime driver’ behind breast development starting earlier although there are a number of theories as to why obesity might be linked to earlier onset of puberty.
The researchers add that maturing at a younger age could come with long-term risks for some girls including higher risk of breast and other cancers because their bodies spend more years making, and being exposed to, oestrogen.
They also tend to start having sex or using drugs and alcohol at younger ages and are more likely to become depressed or develop low self-esteem, Dr Biro added.
"You've got a 10-year-old who looks like a 14-year-old. We interact with kids based on the way that they look,’ Dr Biro said.
"Kids interact with each other that way also. One of the things the parents of these early maturing kids need to do is they need to monitor them more closely."