Dr. Catherine Gordon, a pediatric endocrinologist and specialist in adolescent medicine at Children’s Hospital Boston, said that so far, most evidence showed that neither breast development nor menstrual age had changed for white girls of normal weight.

The new study included 1,239 girls ages 6 to 8 who were recruited from schools and examined at one of three sites: the Mount Sinai School of Medicine in Manhattan, Cincinnati Children’s Hospital or Kaiser Permanente Northern California/University of California, San Francisco. The group was roughly 30 percent each white, black and Hispanic, and about 5 percent Asian.

At 7 years, 10.4 percent of white, 23.4 percent of black and 14.9 percent of Hispanic girls had enough breast development to be considered at the onset of puberty.

At age 8, the figures were 18.3 percent in whites, 42.9 percent in blacks and 30.9 percent in Hispanics. The percentages for blacks and whites were even higher than those found by a 1997 study that was one of the first to suggest that puberty was occurring earlier in girls.

The new study is being published on Monday in the journal Pediatrics. It was paid for by government grants and conducted at hospitals that are part of the Breast Cancer and the Environment Research Centers, a group formed in 2003 after breast cancer advocates petitioned Congress to set aside money to study possible links between environmental exposures and breast cancer.

If there is an ideal age when girls should reach puberty, no one knows what it is, said Dr. Marcia E. Herman-Giddens, a researcher at the University of North Carolina, Chapel Hill. A girl needs a certain amount of body fat to start menstruating, and girls who are malnourished or ill may have delayed puberty.

In developed countries, the age of puberty dropped from the 19th to 20th centuries, as nutrition improved and infectious diseases were brought under better control, and it was seen as a sign of progress. But if the drop continues, at what point does it become pathological?