(CNN) Boys in the United States are starting puberty earlier than ever, according to a new study publishing in the November issue of the journal Pediatrics.

In the study, lead author Marcia Herman-Giddens from the University of North Carolina's School of Public Health and her colleagues show that boys are starting to sexually develop six months to two years earlier than medical textbooks say is standard.

This research has been a long time coming. Herman-Giddens first documented early puberty in girls in 1997, and several studies have since backed up those findings

One of the reasons it took so long to do a comprehensive study on early puberty in boys, Herman-Giddens said, is that the onset is more difficult to identify. For girls, breast development and the start of a menstrual cycle are obvious clues. For boys, the onset of puberty comes in the form of enlarged testes and the production of sperm.

Researchers responded: " 'Yikes, we don't want to ask about that!' " Herman-Giddens said with a laugh.

But ask they did -- 212 practitioners across the country examined more than 4,100 boys aged 6 to 16. The practitioners recorded information on the boys' genital size and pubic hair appearance.

Researchers assigned each boy's data to one of five stages -- Stage 1 being pre-puberty, Stage 2 being the onset of puberty and Stage 5 being adult maturity. They then compared the ages and puberty stages of all the boys. The rigorous study was designed to report on only physical changes, not hormonal.

The results were broken down by race: African-American boys start hitting Stage 2 first, at about 9 years old, while non-Hispanic white and Hispanic boys begin developing around 10 years old. "This should have an impact on the public health community," Herman-Giddens said.

But the researcher is concerned about using the numbers as a new standard for pediatricians. "That might be normal now," she said, "but that doesn't mean it's normal in the sense of what's healthy or what should be."

One of the reasons she's worried is that our environment may be playing a role in accelerating puberty.

"The changes are too fast," Herman-Giddes said. "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."

Dr. Megan Kelsey, an assistant professor of pediatric endocrinology with Children's Hospital Colorado , said several studies have shown an association between childhood obesity and early puberty in girls.

Fat tissue has the ability to convert other hormones into estrogen, which experts believe may lead to early breast development. Fat also creates the hormone leptin, which is necessary for the onset of puberty, Kelsey said.

The little research that has been done on the relation between obesity and puberty in boys has shown conflicting results. A few studies found puberty is delayed in obese males -- possibly because of excess estrogen in the body.

Obesity could still be to blame, but a closer examination is needed, Kelsey said. The problem is a lot of other factors that could be involved.

"It's a very complicated subject," said Sonya Lunder, a senior analyst with the Environmental Working Group . "We're finding a lot of the chemicals that Americans have daily exposure to have an impact."

But identifying the specific chemicals that are causing hormonal changes has so far been impossible. Skepticism abounds about food additives, pesticides and chemicals like BPA, Lunder said.

"The overall concern is that by hastening puberty you're actually shortening childhood," she said. "The real impact of this is not only on future fertility," but also that puberty is a "physiological change in your brain."

Parents should be aware that their boy or girl could hit puberty earlier than they did as children, Herman-Giddes said. They may need to give "the birds and the bees" talk earlier or be prepared to explain their child's body changes.

Early development in girls has been linked to poor self-esteem, eating disorders, and depression, according to Health.com . The findings for boys are not as clear, but parents should be on the lookout for risky behaviors.

One thing that should be a relief for parents, though, is that boys are still seemingly reaching sexual maturity at the same age as they have in the past.

"Although they seem to be starting a little bit earlier, they seem to reach the end of puberty at the same time as they used to," Kelsey said. We're "not going to have grown men in eighth grade."