This article investigates how age diversity within a company's workforce affects company productivity. It introduces a theoretical framework that helps integrate results from a broad disciplinary spectrum of ageing and diversity research to derive empirically testable hypotheses on the effects of age diversity on company productivity. It argues that first the balance between costs and benefits of diversity determines the effect of age diversity on company productivity, and that second the type of task performed acts as a moderator. To test these hypotheses, it uses a large-scale employer–employee panel data set. Results show that increasing age diversity has a positive effect on company productivity if and only if a company engages in creative rather than routine tasks.