High-income countries at present tend to have relatively large and growing aging populations. Conversely, in most low-income countries children and youth account for very large proportions of the population. Notwithstanding these divergent demographic situations, current and projected changes in the composition and distribution of population in high- and low-income countries alike provide opportunities for strengthening weakened social safety nets, promoting economic sustainability, and improving social integration in cities by adopting development strategies that support intergenerational initiatives.