Researchers have found that not just the girls, but even boys are reaching puberty earlier than what is viewed as the standard age.

The researchers found that the boys were reaching puberty six months to two years earlier than the 11 ½ years that was considered to be the benchmark age for the onset of puberty. The researchers also found that African American boys reached puberty faster than Hispanic, non Hispanic or white boys.
Trying to explain the faster onset of puberty, lead researcher Marcia Herman-Giddens said that it could not be due to genetic evolution. “The changes are too fast. Genetics take maybe hundreds, thousands of years. You have to look at something in the environment. That would include everything from (a lack of) exercise to junk food to TV to chemicals”, she said.
Source-Medindia