Boys in the United States may be entering puberty earlier than in generations past, a new study has found, suggesting it’s not just girls who are developing at younger ages.

In comparisons with decades-old data, boys who were seen for well-child visits between 2005 and 2010 were maturing six months to two years sooner, based on their genital development.

The finding is significant for researchers seeking to understand why the age of puberty may be creeping down.

The discovery is also important for parents, who have to know how and when to discuss changing bodies with their children, according to the lead author of the study published online Saturday by the journal Pediatrics.

“They need to talk to their boys earlier than they would have thought about puberty and sexual development and all of those related issues,” said Marcia Herman-Giddens at the University of North Carolina at Chapel Hill.

Recent studies from the United States and elsewhere have shown that girls are maturing at a younger age, with many starting to develop breasts as early as age 7 or 8.

Doctors haven’t necessarily thought the same early puberty trend applied to boys. Some doctors blame estrogen-like chemicals in the environment for girls’ earlier development. Those same chemicals would be expected to delay sexual maturation in boys.

But even if boys are developing earlier than in the past, that doesn’t mean they are more mature socially and psychologically at younger ages, researchers said.

“Now there’s probably a bigger disparity between their physical maturation and their psychosocial maturation,” said Dr. Frank Biro, head of adolescent medicine at Cincinnati Children’s Medical Center, who wasn’t involved in the new analysis.

“People are going to interact with them like they’re older,” he told Reuters Health.

CHANGE MAY NOT BE HEALTHY

Data for the new study came from 144 U.S. pediatric practices and included 4,131 boys age 6 to 16.

Based on the so-called Tanner stages of development - a technique doctors use to measure how far along in puberty a young person is - genital changes in boys started around the age of 9 or 10, and pubic hair appeared between age 10 and 11 1/2, on average.

Testicle size hit a common measure for the start of puberty just before age 10 and full sexual maturity happened at 15 to 16.

In general, African-American boys developed earlier than their white and Hispanic peers, Herman-Giddens and her colleagues reported.

A study from the 1950s through 1970s of white boys in England - for which the Tanner scale is named - found boys started genital development at age 11.6, on average. Other data through the 1970s also put the start of genital changes between age 11 and 12, and pubic hair development typically between 12 and 13 - about two years later than in the new study.

Despite strong evidence showing girls are developing breasts and getting their periods at younger and younger ages, Herman-Giddens said it is still not clear why boys may also be hitting puberty sooner than in years past. One possible explanation is high rates of obesity, which alters the body’s hormone levels.

“The reasons it is happening may not be healthy,” she told Reuters Health.

The new study wasn’t designed to be representative of what is happening across the country. Biro said he thinks puberty is indeed coming earlier in U.S. boys in general, though other researchers may not agree.

Regardless of general trends, it is important for parents to pay attention to their own child’s development, researchers said, and to know when to start talking to them about sexual activity, he said.

“Parents need to monitor both their daughters and their sons a little more closely than they would have before,” Biro said.

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By Genevra Pittman