Analysis of population ageing is typically framed in terms of the number of years lived since birth. This column uses data from studies on physiological functioning in the US and English populations aged 50 and older to show that for policy and research purposes, the reliance on chronological age is at best incomplete and at worst misleading as it provides only limited information about the ageing process. Improvements in how we measure ageing and our understanding of how it affects people and the economy are needed if we are to adapt to the reality of longer lives and overcome the challenges posed by population ageing.