CHICAGO (AP) - A new study suggests that boys in the United States, like girls, are entering puberty slightly earlier than previously thought, with blacks the most likely to develop the first signs by age 10.

Skeptics challenged the findings but said they raise important questions worthy of more comprehensive study.

Early puberty may increase a boy's chances of developing testicular cancer later in life because it may mean longer exposure to sex hormones, said University of North Carolina researcher Marcia Herman-Giddens, the study's lead author. If boys are truly maturing earlier, then sex education classes should begin earlier, she said.

The study - an analysis of a 1988-94 federally funded national health survey - found the average age for developing pubic hair was 12 in white boys, 11.2 years in blacks and 12.3 years for Mexican-Americans. That's up to half a year earlier than in earlier studies, Herman-Giddens said.

But 21 percent of black youngsters studied had developed pubic hair between their 9th and 10th birthdays, compared with 4.3 percent of white boys and 3.3 percent of Mexican-Americans.

The study also suggests that a significant number of boys as young as 8 in all three races had signs of genital development - some three years earlier than previous estimates. But the authors and other experts say those signs likely are too subjective to draw conclusions from.

On average, ages for the start of genital growth were 10 for white boys, 9.5 for blacks and 10.4 for Mexican-Americans.

The study appears in September's Archives of Pediatrics & Adolescent Medicine. Herman-Giddens' previous research, published four years ago, suggested that significant numbers of white and black girls begin to develop sexually by age 8.

Potential reasons for earlier development include rising obesity rates, better nutrition, exposure to environmental chemicals that can mimic sex hormones and use of infant formula and other products containing soy, which also can mimic sex hormones, Herman-Giddens said.

Genetics and environmental differences may explain racial disparities, she said.

Critics said the findings are flawed because assessments of physical changes were based on visual exams by researchers, not the boys' pediatricians.

While the start of pubic hair growth is generally the first sign of puberty, it may also occur in response to hormonal changes that don't necessarily mean sex hormone activity has begun, said Dr. Peter A. Lee, a pediatric endocrinologist at Pennsylvania State University's Hershey Medical Center.

Testicular growth is generally the second stage, but determining whether the testes have enlarged is extremely subjective and most reliable when done by a doctor who has followed a boy's growth for years, Lee said.

The study "does not clearly, unequivocally indicate that puberty is occurring earlier" but underscores "the need for good, reproducible data," Lee said.

Dr. Edward Reiter of Tufts University medical school said even if boys are starting puberty slightly earlier, it's likely no cause for concern, especially since data suggest they're completing puberty about the same time as previous estimates - reaching sexual maturity around fifteen and a half; on average.

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