Talking to your children about sex can be awkward, but new research suggests that parents need to have those conversations much earlier than they do.

In two national surveys, investigators found that between 4% and 8% of boys reported having sex before they were 13. That number varied greatly depending on where the boys lived. In San Francisco, just 5% of boys said they had sex before 13, but in Memphis that number jumped to 25%.

Race and ethnicity also made a difference in whether or not a young person had an early sexual experience. Black males were most likely to have sex before 13, followed by Hispanic males.

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"Parents and educators can't wait until a high school class to cover key topics when many young males start having sex before this," said study author Laura Lindberg, from the Guttmacher Institute in New York City. The study was published April 8 in the journal JAMA Pediatrics.

The author of an accompanying editorial, Dr. David Bell, agreed. "The big picture for me is that we need to make sure our young people are better prepared and better educated around sex," he said.

Bell, an associate professor at Columbia University's Mailman School of Public Health, pointed out that talking about sex doesn't encourage young people to have sex.

The average age that teens initiate sex is around 17, according to Bell, and that number hasn't really changed in years. However, he said that the percentage of youths having early sex initiation (before 13) has been decreasing for more than a decade.

And that decline is good news, because having sex before 13 is linked to increased sexual risks, such as having multiple sexual partners and sexually transmitted infections, the editorial said. It may also be linked to substance abuse, dating violence and low school achievement.

The latest study looked at two large databases that included nationally representative groups. One study included nearly 20,000 high-school aged males. The other had almost 8,000 males between the ages of 15 and 24.

The volunteers were asked if they had experienced male-female sexual intercourse before age 13. Most of those who had sex before 13 said they had sex with a "friend." Lindberg said they didn't ask in this study if the friend was of a similar age, but previous research suggests that when boys have sex early, their partners are often close in age.

Fifty-five percent of the young men said they wanted the sexual experience, and 37% said they had mixed feelings about it. Eight percent said they didn't want it to happen.

Parental education appeared to have an impact. Boys with mothers who had a college degree were 69% less likely to have sex before 13.

As to why there are such variations in early sex rates, Lindberg said, "Adolescent males' attitudes and values about their sexuality and masculinity are influenced by the social context of their community.

"Our findings reflect that where you live exposes you to different social norms about manhood," she added. "The variation across settings means that programs for young people's development and health need to be tailored and responsive to the communities they are in."

Bell said that despite the geographical, racial and ethnic differences seen in the study, all young people need to receive sex education and parents need to be ready to have open, honest talks with their kids.

Bell also said that a child's pediatrician can play a helpful role if parents are struggling. And pediatricians should be aware of these findings because children who might engage in early sexual activity should be offered the HPV vaccine early (to prevent the sexually transmitted disease that causes some cancers), when it is most effective.