The importance of the timing of first intercourse in one's life history, and its significance in relation to a number of fertility and social outcomes, has been established in a number of studies. Studies have attempted to untangle the factors that contribute to its timing, and only some of these studies explore the possibility of selection influences on this outcome. This study uses National Longitudinal Survey of Youth (NLSY) samples and multilevel survival models to evaluate predictors of age at first intercourse (AFI) at both the family and individual level. The family structure among the NLSY samples enables the use of a children of siblings type design so that we may also investigate the possible influence of selection effects. Intelligence and educational goals are often implicated as factors motivating adolescents and young adults to delay AFI. Extended family, maternal, and child intelligence variables are the predictor variables of focus in this study. Other variables include maternal AFI, measures of the home environment, and family income, as these variables also relate to the evaluation of educational goals. Gender and race are also included as control variables. None of the intelligence variables were found to be significant predictors of AFI, though interesting trends emerged. Maternal AFI was consistently a significant predictor across models, but was later identified as non-significant relative to average AFI at the maternal family level. Possible explanations for these findings are offered.