How age groups in the U.S. have shifted between 1950 and 2060

ASPEN, Colo.—Throughout human history, plotting populations by age group has yielded a pyramid. Across societies, the young (the base) exceeded the old (the tip).

But in recent decades, these pyramids have been morphing into rectangles and other previously unseen shapes. Just look at how the age breakdown of the U.S. population is expected to change between 1950 and 2060, in the GIF below. As early as 2030, the percentage of Americans 65 and older could surpass the percentage that is younger than 15.

"This is uncharted water," according to Paul Taylor, who researches demographic and generational changes at the Pew Research Center.

This trend isn't confined to the United States. Between 2010 and 2050, global population growth is forecast not only to slow but also to be strongest among older age groups (the phenomenon is blunted somewhat by rapid population growth in regions such as Africa).

In fact, the shifts are even more pronounced in some European and Asian countries than they are in America. Demographers anticipate that by 2050, the majority of people in countries such as Japan, South Korea, and Germany will be older than 50.

By mid-century, China, in part because of its one-child policy, will have an age pyramid resembling a "top-heavy trapezoid," Taylor said in a talk on Saturday at the Aspen Ideas Festival, which is organized by the Aspen Institute and The Atlantic. In Germany, deaths have exceeded births every year for the last four decades. And the most dramatic manifestation of the phenomenon is in Japan.