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To estimate the association between age at antiretroviral therapy (ART) initiation and immunologic response over time by stratum of baseline CD4+ cell counts.Retrospective cohort analysis of data pooled from four President's Emergency Plan for AIDS Relief funded countries in Sub-Saharan Africa.General linear models were used to estimate the mean CD4+ cell count by age group within groups defined by baseline CD4+ cell count. Kaplan–Meier methods were used to estimate time to achieving a CD4+ cell count of at least 500 cells/μl by age group and stratified by baseline CD4+ cell count.A total of 126 672 previously treatment-naive patients provided 466 482 repeated CD4+ cell count measurements over 4 years of ART. The median baseline CD4+ cell count for all age groups was less than 200 cells/μl. Patients aged 30–39, 40–49, 50–59, and 60 and older at ART initiation had significantly lower mean CD4+ cell counts in most strata and at most time points than those 20–29 years old. Compared with those 20–29, all older age groups had a significantly longer time to, and lower rate of, achieving a CD4+ cell count of 500 cells.Age is associated with the magnitude of CD4+ cell gain and the amount of time it takes to gain cells at different levels of baseline CD4+ cell count. The delay in achieving a robust immune response could have significant implications for the risk of tuberculosis reactivation as well as comorbidities associated with age in the management of older HIV-infected patients.