We estimate the age distribution's impact on carbon dioxide emissions from 1990 to 2006 by exploiting demographic variation in a panel of 46 countries. To eliminate potential bias from endogeneity or omitted variables, we instrument for the age distribution in a country's current population with lagged birth rates, and the regressions control for total population, total output, and country and year fixed effects. Carbon dioxide emissions increase with the share of the population aged 35 to 49 years, and this result is statistically significant and quantitatively large. (JEL Q4, J1)