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 researchesdemographic and generational changes at the Pew Research Center.