Youth who experience adverse environments in early life initiate sexual activity at a younger age, on average, than those from more advantaged circumstances. Evolutionary theorists have posited that ecological stress precipitates earlier reproductive and sexual onset, but it is unclear how stressful environments interact with genetic influences on age at first sex. Using a sample of 1,244 pairs of twins and non-twin full siblings from the National Longitudinal Study of Adolescent Health, the present study tested for gene-by-environment interactions (G × E) on age at first sex (AFS). Multivariate interaction models indicated that genetic influences on AFS were suppressed among low-socioeconomic-status (SES) and ethnic-minority compared with higher SES and ethnic-majority youth. Father absence did not uniquely moderate genetic influences on AFS. These results are broadly consistent with previous findings that genetic influences are minimized among individuals whose environments are characterized by elevated risk; however, future research would benefit from samples with larger numbers of individuals at the very low end of the SES spectrum.