There is a growing awareness of the aging of populations around the world and the implications for national retirement programs. In most cases, estimates of population aging are based on fixed assumptions about fertility, improvements in life expectancy, and immigration. In most countries, however, these factors have varied considerably in recent decades. In this analysis, the authors use population projections in the G-7 countries that capture historical patterns of fertility, longevity, and immigration and variability in those patterns over time. They find that many developed countries may be underestimating the extent of population aging they are facing and the policy changes necessary to deal with it.