For the new study, researchers recruited pediatricians in 41 states who participate in the academy's office-based research network. Doctors asked parents and boys aged 6 to 16 to take part during regular checkups. The visits took place between 2005 and 2010.
Half of the boys were white. The rest were almost evenly divided among blacks and Hispanics.
On average, white boys started puberty at age 10, a year and a half earlier than what has long been considered the normal average. For black boys, the average age of 9 was about two years earlier than in previous research. Among Hispanics, age 10 was similar to previous research that only involved Mexican-American boys. The new study included boys from other Hispanic backgrounds.
Testes enlargement was seen at age 6 in 9 percent of white boys, almost 20 percent of blacks and 7 percent of Hispanics.
Pubic hair growth, another early sign of puberty, started about a year after testes enlargement in all groups but still earlier than previously thought.
In girls, breast development is the first sign, and recent research suggested that it starts at age 7 in about 10 percent of white girls, 23 percent of blacks and 15 percent of Hispanics. That's substantially higher than rates reported more than a decade ago.
Read more at the Associated Press.