By Amy Norton

HealthDay Reporter

MONDAY, Nov. 11 (HealthDay News) -- Girls who start menstruating at a younger-than-average age do not have sex earlier than their peers, according to a new study.

Australian researchers found that among 554 girls followed through childhood, those who started their monthly periods before age 12 did not get an earlier start on their sex lives.

One-quarter had had sexual intercourse by the time they were 15 years old, versus about 29 percent of girls who'd started their menstrual periods at age 12 or 13 -- which is considered the average age range.

Typically, the study found, girls who got their first period early waited longer before having sex -- about five years versus less than four years among girls who started menstruating at the average age.

"Our study shows that earlier physical maturation doesn't necessarily mean a younger girl will initiate sexual behavior," said lead researcher Jennifer Marino, a research fellow at the University of Melbourne in Australia.

Some past studies have linked earlier menstruation to earlier sexual activity. But, Marino said, those have generally been of older girls and women who were asked to recall how old they were when they started their periods and started having sex.

In this study, girls were followed from childhood to age 20, and they and their parents answered periodic questionnaires; some girls kept diaries to record their periods.

Still, the findings, reported online Nov. 11 in Pediatrics, do not prove there's no link between earlier puberty and earlier sexual activity, one expert says.

"The age of menarche and the age of onset of puberty are not the same thing," said Dr. Renee Boynton-Jarrett, an assistant professor of pediatrics at Boston University School of Medicine who studies pubertal development.

"Menarche" refers to a girl's first menstrual period, and that actually happens late in puberty, Boynton-Jarrett explained. For girls, the first sign of puberty is the beginning of breast development.

"In the U.S., the age of onset of breast development is declining," Boynton-Jarrett noted. "And it would be interesting to see if the age of pubertal onset is associated with first sexual activity."

Just last week, a U.S. study found that black girls typically start developing breasts at about age 8, while other girls start at around age 9. The researchers found that among white girls, breast development is happening sooner now than in the 1990s -- and a rising rate of obesity seemed to largely explain the shift.

It's important to understand whether that earlier development is related to earlier sexual activity, Boynton-Jarrett said. If it is, she noted, it might be wise to start sex education in schools a bit sooner.

Another limit of the current study is that it looked at a group of mostly white girls from Western Australia, and it's not clear if the findings would extend to more diverse groups, said Boynton-Jarrett.

Marino agreed. "It's possible that the U.S., which has a different racial, ethnic and cultural composition than Australia, could have different findings," she said. Race and ethnicity influence the timing of puberty, Marino noted, and a whole host of factors -- including family and social influences -- would affect girls' sexual activity.

But that's also good news for parents. Even if earlier puberty were to be generally linked to earlier sex, that would be far from the only factor in any one girl's odds of becoming sexually active at a young age, Boynton-Jarrett said.

Regardless of when a child starts physically maturing, parents should talk to their kids about sex, Marino said.

"Although some people think that teaching kids about sex will 'give them ideas,' decades of data show otherwise," Marino said. "Starting conversations early makes it more likely that teenagers will talk to their parents if they have questions later on, or find themselves in a tricky situation."

Most of the research on the timing of puberty and sexual activity has focused on girls. Judging the stages of puberty in boys is simply more difficult from a practical standpoint, Marino noted.

But, she said, a couple of small studies involving boys have suggested a link between earlier puberty and earlier sexual activity.

More information

The American Academy of Pediatrics has tips on talking to your kids about sex.