From the New York Times:
A large study released by the American Academy of Pediatrics suggests that boys are entering puberty earlier now than several decades ago — or at least earlier than the time frame doctors have historically used as a benchmark.
The study, widely considered the most reliable attempt to measure puberty in American boys, estimates that boys are showing signs of puberty six months to two years earlier than was reported in previous research, which historically taught that 11 ½ was the general age puberty began in boys. But experts cautioned that because previous studies were smaller or used different approaches, it is difficult to say how much earlier boys might be developing.
The study echoes research on girls, which has now established a scientific consensus that they are showing breast development earlier than in the past.