We used an established microsimulation model, quantified to a rural South African setting with a well developed antiretroviral treatment programme, to predict the impact of antiretroviral therapy on the HIV epidemic in the population aged over 50 years. We show that the HIV prevalence in patients aged over 50 years will nearly double in the next 30 years, whereas the fraction of HIV-infected patients aged over 50 years will triple in the same period. This ageing epidemic has important consequences for the South African healthcare system, as older HIV patients require specialized care.