A “population pyramid” is a graph that shows the number of people of each age group divided into male and female.

Since the oldest age groups at the top of the figure have small populations, the graph will narrow toward a point at the top. But looking across population pyramids for different years, you can see the movements of larger and smaller generations as they age. Here’s are population pyramids for 2000, 2010, and 2020 from the US Census Bureau (“Age Profiles of Smaller Geographies Don’t Always Mirror the National Trend,” by Laura Blakeslee, Megan Rabe, Zoe Caplan and Andrew Roberts, May 25, 2023).

The authors write:

The pyramid was larger in 2020 than it was in either 2010 or 2000. This reflects the growth in the U.S. population: 331.4 million people in 2020, up 22.7 million (7.4%) from the 308.7 million in 2010. Between 2000 and 2010, the population grew by 27.3 million (9.7%) from 281.4 million people. The U.S. population also aged since 2000. The baby boom cohort moved up the pyramid, from 36-to-54-year-olds in 2000 to 46-to-64-year-olds in 2010 and 56-to-74-year-olds in 2020. The millennials were mostly in their teens and 20s in 2010 but young adults (in their 20s and 30s) a decade later. At the same time, the base of the pyramid representing children under age 5 got smaller in 2020, reflecting a recent decrease in the number of births in the United States.

Population pyramid diagrams have been around a long time. The authors also include one from a Bureau of the Census report in 1900. Again, each bar represents five years of age. You can see that the number of people in the 85-90 age group almost disappears in this diagram, and the bars for those 90 and over do disappear. You can also see that this is a true “pyramid,” in the sense the younger the age group, the bigger it is. Of course, this is the sign of a growing population.