In this paper, we argue that entry into first sexual intercourse is a key process mediating the effects of family structure on premarital childbearing. We explicate three ways in which onset of sexual activity can mediate effects of family structure on premarital first births. First, the gross association between family structure and premarital birth risks may be due entirely to the effect of family structure on age at first intercourse. Second, the earlier the age at first intercourse, the longer the duration of exposure to the risk of a premarital first birth. Third, an early age at first intercourse may proxy unmeasured individual characteristics correlated with age at onset but uncorrelated with other variables in the model. We develop methods to assess such mediating effects and analyze data from two sources, the 1979-93 National Longitudinal Survey of Youth and the 1988 National Survey of Family Growth. We find that age at first intercourse partially mediates the effect on premarital birth risks of both snapshot measures of family structure at age 14 and a time-varying measure of the number of family transitions, but that significant effects of these variables remain net of age at first intercourse. Delaying age at intercourse by one year reduces the cumulative relative risk of a premarital first birth by a similar amount for both white and black women. For black women, the magnitude of this effect is roughly the same as that of residing in a mother-only family at age 14.