CHICAGO (Reuters) - Teenagers who have had formal sex education are far more likely to put off having sex, contradicting earlier studies on the effectiveness of such programs, U.S. researchers said on Wednesday.

They found teenage boys who had sex education in school were 71 percent less likely to have intercourse before age 15, and teen girls who had sex education were 59 percent less likely to have sex before age 15.

Sex education also increased the likelihood that teen boys would use contraceptives the first time they had sex, according to the study by researchers at the U.S. Centers for Disease Control and Prevention, which was published in the Journal of Adolescent Health.

"Sex education seems to be working," Trisha Mueller, an epidemiologist with the CDC who led the study, said in a statement. "It seems to be especially effective for populations that are usually at high risk."

Mueller's team looked at a 2002 national survey of 2,019 teens aged 15 to 19.

They found teen boys who had sex education in school were nearly three times more likely to use birth control the first time they had intercourse. But sex education appeared to have no effect on whether teen girls used birth control, the researchers found.

Black teenage girls who had sex education in school were 91 percent less likely to have sex before age 15.

The researchers did not evaluate the content of sex education programs, including whether students were taught about contraception or about abstinence only.

Earlier studies, which relied on data from the 1970s through the 1990s, suggested sex education did little to persuade teens to delay sex.

The researchers said they think the difference may be that sex education in the United States is now more widespread and is being taught at earlier ages.

"Unlike many previous studies, our results suggest that sex education before first sex protects youth from engaging in sexual intercourse at an early age," they wrote.