Over time, the decline in death rates has become increasingly concentrated at older ages. People are not only more likely to survive childhood, they are more likely to survive well into retirement. While improvements in mortality rates at older ages have led to more rapid population aging, the most important factor leading to an increase in the share of the old- age population is low fertility. The share of the old- age population in countries like Japan, Republic of Korea, and Singapore is projected to reach high levels because low fertility rates will lead to substantial declines in the working- age population.