NEW YORK - People who start having sex at a younger or older than average age appear to be at greater risk of developing sexual health problems later in life, a new study suggests.

The findings, according to researchers, cast some doubts on the benefits of abstinence-only sexual education that has been introduced in U.S. public schools.

Using data from a 1996 cross-sectional survey of more than 8,000 U.S. adults, the researchers found that those who started having sex at a relatively young age were more likely to have certain risk factors for sexually transmitted diseases (STD) -- including a high number of sexual partners and a history of having sex under the influence of alcohol.

On the other hand, both "early" and "late" starters were at increased risk of problems in sexual function. This was true primarily of men, whose problems included difficulty maintaining an erection and reaching orgasm. The findings are published in the American Journal of Public Health.

It's not clear from the survey why both early and late starters tend to have more sexual dysfunction, according to the researchers, led by Dr. Theo G.M. Sandfort of Columbia University in New York City.

But the findings, they write, "only partially support" abstinence-only sex education -- which encourages teenagers to save sex for marriage.

"Although our findings support an association between early initiation and long-term (STD) risk, they also suggest a more complicated picture of sexual functioning," Sandfort and his colleagues write.

Delaying sexual activity may "create health risks by impeding development of the emotional, cognitive, and interpersonal skills that are crucial to satisfactory sexual functioning and general well-being," they add.

On average, respondents said they had sex for the first time around the ages of 17 or 18. Those who had their first sexual encounter at average age of 14 were considered "early starters" and those who started at age 22 or older were considered "later starters."

It's not possible to determine cause-and-effect from the survey data, according to the researchers.

For example, young men with sexual problems may start having intercourse at a later age, contributing to the link between later sexual "debut" and higher odds of sexual dysfunction.

However, Sandfort's team adds, the findings lend credence to other studies suggesting that abstinence-only education may actually increase the risk of certain health problems.

"Sexual education that is more supportive and acknowledges the diverse needs of young people might prevent the negative outcomes observed here," the researchers write.

SOURCE: American Journal of Public Health, January 2008.