Democracies have experienced profound population ageing in the last decades. Yet we still know little about the political consequences of ageing for economic performance. In this article, I develop a novel theoretical framework linking ageing to lower economic growth through two mechanisms: first, grey power pushes elected governments to expand old age policies thereby ‘crowding out’ more growth-enhancing policies; second, ageing populations weaken the electoral penalty for lower economic performance leading to ‘economic unaccountability’. Using microdata from four cross-national survey of preferences and vote choices, I show that elderly individuals care more about pensions, but less about education, childcare and family policies, and during elections they are less likely to penalize governments for low growth. Using macrodata on 21 advanced economies since the 1960s, OLS and instrumental variable regressions provide evidence that ageing leads to more spending on pensions policies but less on education, family, and childcare policies, as well as public investments, and lower growth. Ageing countries may paradoxically become economically inefficient because they are politically representative.