Background With the scale-up of antiretroviral treatment across Africa, many people are living longer with HIV. Understanding the ageing of the HIV cohort and sexual behaviour among older adults are important for appropriately responding to the changing demographics of people living with HIV. Methods We used data from a large population-based open cohort in eastern Zimbabwe to examine HIV prevalence trends and incidence among those aged 45 years and older. Five survey rounds have been completed between 1998 and 2011. Incidence was analysed using midpoint between last negative and first positive HIV test. Results Across the survey rounds, 13,071 individuals were followed for 57,676 person years. While HIV prevalence among people aged 15???44 has fallen across the five rounds, HIV prevalence among those aged 45???54 has increased since the 2006???08 survey round. In the 2009???11 round, HIV prevalence among men aged 45???54 was 23.4% compared to 11.0% among those aged 15???44. HIV positive people aged 45???54 now represent more than 20% of all those living with HIV in Manicaland. Among those aged 45 years and older, there were 85 seroconversions in 11,999 person years for an HIV incidence of 0.708 per 100 person years. Analysis of cohort data and assessment of behavioural risk factors for HIV infection among older people shows significantly lower levels of condom use among older adults and a number of seroconversions past the age of 50. Conclusions The cohort of people living with HIV is ageing in Zimbabwe and the behaviour of older adults puts them at risk of HIV infection. Older adults must be included in both HIV prevention and treatment programs.