“It was an important study to do, and their 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: the size of the testicles,” said Dr. Laura Bachrach, a professor of pediatric endocrinology at Stanford University.

But the study should not prompt a magazine “cover article that shows a 9-year-old boy shaving,” Dr. Bachrach said. And because some parents fear that early puberty is related to more hormones in milk — speculation that is unproven — “I don’t want people to get up in arms and rush out and buy organic milk,” she said. “When patients ask me, I say, ‘Do that for political reasons or because you like the taste, but don’t do it because you think it’s going to influence puberty.’ ”

For the study, researchers enlisted about 200 pediatricians in 41 states to record information on 4,131 healthy boys ages 6 to 16 during their well-child exams. Physicians were trained to use an orchidometer, a string of oval wooden or plastic beads of increasing size that are compared against the size of the testicles. Urologists use orchidometers to measure testicular volume when men have fertility concerns. Normal adult size is about 22 to 25 milliliters, Dr. Lamb said. In boys, 2 milliliters is pre-pubertal; some doctors consider 3 milliliters and others 4 milliliters as an indicator of puberty, so the study included analysis for both sizes.

Doctors in the study also evaluated boys using the Tanner scale, a five-stage ranking system developed from a 40-year-old British study. While Tanner is the textbook benchmark, doctors increasingly consider it outmoded because it involved only 228 white boys in juvenile detention in London and evaluated them from photographs.

In the new study doctors also took note of pubic hair, but, said Dr. Bachrach, “pubic hair is very very misleading” because it is a later, less predictable indicator.

The study’s lead author, Marcia E. Herman-Giddens, a child and maternal health specialist at University of North Carolina Chapel Hill, said she originally proposed an additional measure used in Europe: identifying through urinalysis, or by asking, if boys had begun to ejaculate. But she said that urinalysis would have made the study more expensive, and colleagues reviewing the proposal “just freaked out” about the prospect of asking boys about ejaculation, wondering, “How would a child understand that?”

Dr. Herman-Giddens led a large study on girls’ puberty in 1997, and its conclusion that girls were developing earlier generated great controversy. Now, though, experts generally agree that subsequent research has shown breast development as young as 7 or 8. With menstruation, however, studies conflict: some suggest it is starting earlier, while others suggest the age has not changed much. Experts said this could mean that puberty is beginning sooner but lasting longer, or that different physiological processes underlie breast development and menstruation.