BACKGROUND: Nearly all population-level research showing positive associations between age-disparate partnerships and HIV acquisition among adolescent girls and young women (AGYW) has classified age disparity as >/=5 or >/=10 years. We describe variations in one-year risk of HIV infection after exposure to sexual partner(s) of continuous age disparities. METHODS: Longitudinal data from the HPTN 068 randomized trial in South Africa was used to estimate one-year risk of HIV infection at various age pairings. The parametric g-formula was employed to estimate risk at up to five annual time points, stratified by maximum partner age difference, maximum partner age, and AGYW age. RESULTS: AGYW reported an older partner in 86% of 5,351 age pairings. The one-year risk of HIV infection rapidly increased with maximum partner age difference among girls ages 13 to 14, from 0.01 with a same-age partner, to 0.21 with a partner ten years older, and 0.24 with a partner 15 years older. A gradual increase occurred among AGYW ages 15 to 16, up to 0.13 with a partner 15 years older, and 0.09 among AGYW 17 to 18 with partners 8 to 11 years older. Risk of HIV infection among AGYW ages 19 to 21 remained relatively constant across maximum partner age differences. CONCLUSION: Age differences between AGYW and their sexual partners have a greater effect on HIV risk infection in younger compared to older AGYW. Considering both the age of an AGYW and her sexual partners provides granular insight into identifying key groups for HIV transmission prevention efforts.