Background:
National surveys of sexual behavior in the United States are limited.

Objective:
To determine estimates of several measures of sexual behavior among a national sample of U.S. males and females.

Method:
Data were analyzed from the 1999-2002 National Health and Nutrition Examination Survey (NHANES), a nationally representative survey of the non-institutionalized U.S. population. Measures of sexual behavior were collected using Audio Computer-Assisted Self-Interviewing (ACASI) among 8,856 males and females aged 14-59 years. Sex was defined as either vaginal, oral, or anal sex.

Result:
Among 14-19 year-olds, 43% of males and 50% of females had ever had sex. At 16 years of age, 39% of males and 42% of females had had sex, increasing to 75% of males and 78% of females at age 19 years. Among teenagers who had had sex, mean age at first sex was 14.6 years for males and 15.0 years for females, and median number of lifetime sex partners was 2.3 for males and 1.5 for females. Among 20-59 year-olds, age at first sex was earlier among younger age groups: 33% of 20-29 year-olds first had sex before age 16 years versus 20% of 50-59 year-olds. Median number of lifetime sex partners was 6.7 for males and 3.6 for females, and peaked in the 30-39 year-old age group (7.7 partners for males, 4.3 partners for females). Overall, 4.5% of 18-59 year-old men reported having had a same-sex sex partner, versus 5.0% of 18-59 year-old women.

Conclusion:
The majority of U.S. teenagers have had sex by age 19 years, and age at first sex continues to decrease among younger age cohorts.

Implications:
Sexually transmitted infection prevention efforts should reach younger age groups to target those at risk.