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 1-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 were used to estimate 1-year risk of HIV infection at various age pairings. The parametric g-formula was used to estimate risk at up to 5 annual time points, stratified by maximum partner age difference, maximum partner age, and AGYW age. RESULTS: AGYW reported an older partner in 86% of 5351 age pairings. The 1-year risk of HIV infection rapidly increased with maximum partner age difference among girls ages 13-14 years, from 0.01 with a same-age partner, to 0.21 with a partner 10 years older, and 0.24 with a partner 15 years older. A gradual increase occurred among AGYW ages 15-16 years, up to 0.13 with a partner 15 years older, and 0.09 among AGYW 17-18 years with partners 8-11 years older. Risk of HIV infection among AGYW ages 19-21 years remained relatively constant across maximum partner age differences. CONCLUSIONS: Age differences between AGYW and their sexual partners have a greater effect on HIV-risk infection in younger compared with 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.