The authors examined the relation between age at first vaginal intercourse and a positive nucleic acid amplification test for sexually transmitted infection (STI). A nationally representative sample of 9,844 respondents aged 18-26 years was tested for chlamydial infection, gonorrhea, and trichomoniasis in wave 3 (2001-2002) of the National Longitudinal Study of Adolescent Health. The authors used multiple logistic regression to assess the relation between age at first sexual intercourse and these STIs and to examine variation by current age, sex, race, and ethnicity. Younger ages at first intercourse were associated with higher odds of STI in comparison with older ages, but the effect diminished with increasing current age. For example, the odds of having an STI for an 18-year-old who first had intercourse at age 13 were more than twice those of an 18-year-old who first had intercourse at age 17 (prevalence odds ratio = 2.25, 95% confidence interval: 1.42, 3.59). In contrast, the odds of having an STI among 24-year-olds with first intercourse at age 13 versus those with first intercourse at age 17 were the same (prevalence odds ratio = 1.11, 95% confidence interval: 0.88, 1.39). Thus, earlier initiation of sexual intercourse is strongly associated with STIs for older adolescents but not for young adults over age 23 years.