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Little is known about the relative importance of genetic and environmental factors as determinants of age at first sexual intercourse. In this study, subjects were 5,080 individuals from the Australian Twin Registry (3,310 females, 1,770 males; age range: 27-70 years, median: 40 years) who completed a semistructured interview by telephone in 1992-1993. Self-reported age at first intercourse correlated higher for identical (monozygotic) twins than for nonidentical (dizygotic) twins. Structural equation model fitting found that the genetic contribution to variance was considerably greater among twins aged 40 years or less (72% for males and 49% for females) than for those aged from 41 to 70 years (0%for males and 32% for females). Among the older cohort, there was evidence that somewhat different aspects of the shared social environment influenced age at onset in males and females. In a more laissez-faire social climate in recent decades, it is likely that biological and psychological characteristics that are partly under genetic control significantly influence the age at which a person commences sexual activity., Age at first sexual intercourse is probably greatly influenced in modern times by psychological and biological factors that are partly controlled genetically because social constraints are less than before. Influences on the age of commencement of sexual activity appear to have been different among research subjects aged 41- to 70-years-old than for those aged under 40. Age at first intercourse correlated lower for nonidentical twins than identical twins, with the variance more genetically affected for twins aged under 40 than for those aged 41-70.