This is what demographers call an “age pyramid.” Each bar represents a five year age cohort; with those ages 0-4 on the bottom and those ages 85 and older on the top. In every society since the start of history, whenever you broke down any population this way, you’d always get a pyramid.
But from 1960 to 2060, our pyramid will turn into a rectangle. We’ll have almost as many Americans over age 85 as under age 5. This is the result of longer life spans and lower birthrates.
See more related to the aging population in “The Next America.”