Aria Amour Hill, seen here soon after she was born in Washington, D.C., in 2011, a few days before the world’s population which was estimated to reach 7 billion. (Linda Davidson/THE WASHINGTON POST)

Oct. 31, 2011 The human population hit 7 billion on this day, according to the United Nations, "and an expanding percentage of those people are in the market for reading glasses," The Post's Joel Achenbach observed at the time. "The aging of the human race has been faster than anyone could have imagined a few decades ago. Fertility rates have plunged globally; simultaneously, life spans have increased. The result is a re-contoured age graph: The pyramid, once with a tiny number of old folks at the peak and a broad foundation of children, is inverting. In wealthy countries, the graph already has a pronounced middle-age spread." Longer life spans are a blessing, due in part to advancements in medicine and nutrition. But the graying population was also a factor in the last financial crisis and will continue to challenge certain countries as the ratio of workers to retirees decreases. Achenbach speculated that in the future, more workers may have to work past the traditional retirement age, countries may have to compete more for immigrants, and vast numbers of people may migrate across the planet, from high-fertility countries to low-fertility ones.

Annys Shin