While we're not likely going to see hirsute five-year-olds who need a shave, researchers say boys are hitting puberty as early as 9 and 10 years of age.

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he research question: The scientific consensus is that girls are hitting puberty earlier than ever, developing breasts as young as 7 or 8. Experts credit higher levels of childhood obesity (body fat is linked to estrogen production) and chemicals in our food and water. Is the same trend affecting boys? 

How the question was tested: By focusing on testicle size. The conventional wisdom is that boys hit puberty around 11½. For this study, published in the Journal of Pediatrics, researchers looked at the records of 4,131 boys ages 6 to 16 in 41 states. The data was compiled between 2005 and 2010. "It was an important study to do, and [the researchers'] methodology is improved over prior studies in that they based their assessment of puberty in boys on what I consider to be the gold standard," says Dr. Laura Bachrach, a professor of pediatric endocrinology at Stanford University: "the size of the testicles." 

The outcome: The researchers discovered that, on average, boys were starting puberty almost two years earlier than expected. White and Hispanic boys started puberty at age 10, or a year and a half earlier than what was considered "average." African-American boys hit puberty around age 9 — two years earlier. What's more: Roughly 9 percent of whites experienced testes enlargement as early as age 6. For Hispanic boys it was 7 percent, and for blacks it was nearly 20 percent. Across all groups, pubic hair growth started a year after testes enlargement.

What's behind it: The research wasn't focused on determining an underlying cause. However, childhood inactivity, environmental factors, and changes in diet were all mentioned as possible factors. 

What the experts say: "This should perhaps set a standard going forward for being very attentive to puberty in boys and being mindful that they're developing earlier," says Dolores J. Lamb, a molecular endocrinologist at Baylor College of Medicine and president of the American Society for Reproductive Medicine, who wasn't involved in the study. On the contrary, says lead author Marcia Herman-Giddens, a researcher at the University of North Carolina in Chapel Hill. The results shouldn't be interpreted to establish a "new normal," says Herman-Giddens. "Just because this is happening doesn't mean this is normal or healthy."

The lesson: Something is behind the change — we just don't know what, at least not yet. The findings mean parents will have to "start having the sex talk when their kids are still getting jazzed about Saturday morning cartoons," says Doug Barry at Jezebel. "If kids are looking older [earlier], it means parents should be monitoring them, because that superego doesn't kick in until late teens or early 20s," says Dr. Frank M. Biro, a puberty researcher at Cincinnati Children's Hospital. "The kids need a hand."