Various authors have suggested the possibility of a link between biological age and sexual activity. Zelnick (1981) described the relationship between sexual activity and age at menarche. To determine the relationship between biological age and age at first sexual intercourse, we presented a questionnaire to 450 students attending a comprehensive secondary school in Montreal. Age at menarche and age at first conscious ejaculation were used to measure biological age. A partial correlation was made between biological age and age at first sexual intercourse by controlling for chronological age. The results were r = 0.24 (p <0.029) for female students and r = 0.21 (p <0.08) for male students. The value of the correlation was weak but significant where the female students were concerned. For the male students, the value of the correlation could not be determined due to the insufficient number of respondents. Biological age would appear to be an indicator of the age at which sexual activity first occurs. This indicator could be used by clinicians directly, and to develop a more general model for determining the age at which sexual activity first occurs.