This is a mesmerizing little animation created by Bill McBride of Calculated Risk. It shows the distribution of the U.S. population by age over time, starting at 1900 and ending with Census Bureau forecasts between now and 2060.
Watch what happens to the bottom axis on the right in 1940 and 1990.
Watch the baby bust emerge in 1930.
Watch the baby boom start spiking in 1945 and peak in 1960.
Watch how, around 1990, almost all the cohorts start to flatten out so that each has about 6 to 7 percent of the population.
Disclaimer: The Canadian population did vary from the American experience, such as in the size of the echo boom. It would be great to have a comparative chart.