PURPOSE: To describe variations in age differences (ADs) between partners at first heterosexual intercourse and to examine associations between this AD, sociodemographic characteristics, and circumstances of first heterosexual intercourse, by gender. METHODS: A total of 3277 men and 4734 women aged 25-44 years who reported ever having heterosexual intercourse in a probability survey of the British population to obtain an unbiased sample of ADs at first intercourse. Face-to-face interviews collected sociodemographic data and information on first intercourse. Gender-specific cumulative distributions of ADs categorized respondents reporting relatively younger first partners (below fifth percentile) and those reporting relatively older first partners (above 95th percentile). RESULTS: Relatively older first partners were six years older than men and 10 years older than women. Relatively younger first partners were three years younger than men and one year younger than women. Men and women with relatively older first partners were more likely to have just met their partner (odds ratios adjusted for sociodemographic characteristics [AOR] 1.8, 95% confidence interval [CI] 1.2-2.7, and 3.1, 95% CI 2.1-4.6, respectively); had a more willing first partner (AOR 2.1, 95% CI 1.2-3.8, and 1.6, 95% CI 1.2-2.2, respectively); not used condoms at first sex (AOR 1.9, 95% CI 1.2-3.0, and 1.5, 95% CI 1.1-2.0, respectively); and reported a non-autonomous reason for first sex (AOR 1.6, 95% CI 1.1-2.4, women only). Men with relatively younger first partners were more likely to regret the timing of the first occasion (AOR 2.1, 95% CI 1.0-4.5). CONCLUSIONS: First partnerships involving relatively older or younger partners are associated with adverse circumstances of first intercourse. Condoms and other reliable contraception are less likely to be used, and age differences may reflect unequal power relations. Promoting effective communication and negotiation skills may be particularly important for those with relatively older or younger first partners.