For adolescents, earlier age of sexual initiation is tied to a dramatically higher risk of contracting a sexually transmitted infection (STI), according to a new study in South Korea.

"We are suggesting parents should worry about how and why (kids) would have sex at such an early age, not just the fact they are doing it," said lead author Seo Yoon Lee of the Institute of Health Services Research at Yonsei University in Seoul.

In Korea, parents are avoiding the conversation about sex and the content of sex education has not changed over the last 10 years, even as kids have more and more access to sexual materials online at younger ages, Lee told Reuters Health by email.

The researchers used data from a national web-based survey of youth risk behaviors administered annually by the Korean Centers for Disease Control and Prevention .

A total of more than 525,000 kids in seventh to 12th grades completed the survey between 2007 and 2013. Only 22,400 of those who responded, about 4 percent, said they had experienced sexual intercourse, and only these responses were considered for the new study.

The teens also reported when they first had intercourse, with answers ranging from before elementary school to twelfth grade.

About 7 percent of the teens who had sex said they had experienced an STI, such as gonorrhea, syphilis, chlamydia, HIV infection or other infection.

As age at first intercourse went down, the proportion of teens who had experienced an STI went up, especially for boys, according to the results in the Journal of Sexual Medicine. Compared to kids whose first intercourse was in twelfth grade, for instance, those whose first experience was in seventh grade were three times more likely to have had an STI.

"We think that students or kids who have earlier sexual debut, without proper sex education, would more likely to have riskier sex," Lee said. "Also, earlier sexual debut means their sexually active period becomes longer and during that period, they would have multiple sex partners."

In another study from 2012, South Korean teens had their first sexual experiences around age 15, on average. In this study, however, several hundred teens surveyed claimed that they first had sex before elementary school or in first or second grade, which corresponds to an age of 9 years or younger.

For kids reporting first intercourse in second grade, risk of STI was nine times higher for boys and seven times higher for girls compared to first intercourse in twelfth grade.

"This very young age at sexual intercourse for some youth is a surprising finding and begs the question of whether responses are, in fact, accurate," said Patricia A. Cavazos of Washington University School of Medicine in St. Louis, Missouri, who was not involved in the study.

"Follow-up questions that garner additional information about the event could help to determine the accuracy of the responses," Cavazos told Reuters Health by email. "If the responses are deemed to be accurate, then there should still be uncertainty about whether or not the youth consented to the sexual experience at such a young age."

The researchers honed in on 4 percent of the total sample of teens to address their research questions, so the study findings account for a relatively small percentage of the Korean youth population, she added.

"We, too, think those ages are extremely young," Lee said. The survey may be overestimating the true number of kids having sex that young, or some of them could have been very mature kids with older aged partners such as middle schoolers or high schoolers.

"Or it is possible to think of the abuse too, but again, it is very hard to tell why," Lee said.

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Earlier sexual debut has been tied to other risky health behaviors like unprotected sex, which increases the risk of STIs, and riskier choices in general, Cavazos said.

"Parents should be mindful of this possibility and act accordingly," she said. "They should dialogue with their child about sexual education and the importance of delaying this event until it can be experienced in a responsible and safe way."