Latin American countries are in the midst of a demographic transition and, as a consequence, a population-aging process. Over the next few decades, the number of children will decline relative to the number of older adults. Population aging is the result of a slow but sustained reduction in mortality rates, given increases in life expectancy and fertility. These trends reflect welcome long-term improvements in welfare and in economic and social development. But this process also entails policy challenges: many public institutions—including education, health, and pension systems and labor market regulations—are designed for a different demographic context and will need to be adapted. When We're Sixty-Four discusses public policies aimed at overcoming the two main challenges facing Latin American countries concerning the changing demographics. On one hand, older populations demand more fiscal resources for social services, such as health, long-term care, and pensions. On the other, population aging produces shifts in the proportion of the population that is working age, which may affect long-term economic growth. Aging societies risk losing dynamism, being exposed to higher dependency rates, and experiencing lower savings rates. Nonetheless, in the interim, Latin American countries have a demographic opportunity: a temporary decline in dependency rates creates a period in which the share of the working-age population, with its associated saving capacity, is at its highest levels. This constitutes a great opportunity in the short term because the higher savings may result in increases in capital endowment per worker and productivity. For that to happen, it is necessary to generate institutional, financial, and fiscal conditions that promote larger savings and investment, accelerating per capita economic growth in a sustainable way.