Why are populations of rich countries so much older today than they were a century or two ago? Why are they continuing to age and why are many developing‐country populations expected to age in the twenty‐first century? Is the main driver falling mortality and lengthening life, or is it falling birth rates and slowing population growth? Common sense suggests that longer life is responsible for population aging, but decades ago the work of demographers such as Coale (1956, 1957) and Keyfitz (1975) persuaded us that in fact fertility decline was far more important. More recently, a new wave of demographic analysis suggests that mortality decline is the main demographic source of continuing population aging. Here we revisit this question.