For almost all of human history, the Earth’s population has skewed younger. But since the last World Population Day on July 11, a major shift occurred: There are now more people age 65 and older than there are under age five.

World Population Day was established by the United Nations Development Program in 1989 to bring attention to population issues. Having more people on the planet is not the only concern, though, since a population’s age structure matters too.

Increased longevity is a remarkable human success story, but having more elderly people also creates a number of pressing socioeconomic concerns. The global population will continue to age as these two groups grow in opposite directions. By 2050, the proportion of