Writing in The Atlantic in 2011, Adam Pasick delivered the startling factoid that sales of adult diapers in Japan were set to exceed that of children’s diapers by the year 2020. According to Japan’s biggest diaper maker, he added, the transition actually happened in 2011.

That Japan’s population is aging is hardly a secret. But the speed with which it is happening was laid bare on Friday when the country’s census revealed the population had shrunk by nearly a million people in just five years. As many noted, it was the first decline since Japan’s census started in the 1920s.

“Some areas have felt the dips more keenly than others,” wrote Amy Wang at Quartz. “While Tokyo and seven other prefectures saw an uptick in population, the remaining 39 saw the opposite; Fukushima, hit by an earthquake and tsunami in 2011, posted the biggest loss—roughly 115,000 people.”

Of course, Japan is not alone. Plenty of other countries, particularly across Europe and Asia, are aging. The global population is getting older as well. In the United States, after two decades of hovering above or at two births per woman, the birthrate dipped below that threshold in 2010 and has continued to drop ever since.

As my colleague Olga Khazan noted in 2014, the difference is that America remains a nation of immigrants, which helps keep the country (and its workforce) young.

Immigrants not only help inflate our overall population, but they also tend to have more children than Americans do. Mexican-American women, for example, have 2.5 children on average, and white American women have 1.8. And it’s immigrants who will contribute the most to U.S. population growth in coming decades.

Japanese officials, who have trudged down an increasingly nationalistic path in recent years, have touted the country’s homogeneity. Accordingly, immigration has been limited, and the foreign population, as Joshua Keating points out, is only 2 percent.

That isn’t expected to change dramatically soon, either. According to the government, 40 percent of the country will be 65 years or older by the year 2060.