Even in an unlikely "high growth rate" scenario, America's population has grown at the slowest rate since at least the 1930s, according to recent Census Bureau projections for the last decade.

Why it matters: America is aging. There is a growing number of people out of the workforce, and a relatively smaller number of people trying to support them — a situation that could cripple programs like Social Security and slow economic growth.

• In the lowest growth rate scenario, the U.S. could see the slowest 10-year increase in its population since at least the 1790s, according to analysis by Brookings Institution demographer William Frey.
• Other new Census data found that between July 2019 and July 2020, the nation grew at the lowest yearly rate since at least 1900, largely because of the pandemic.

Americans aren't having as many babies as they used to, mirroringa trend in many developed countries.

• The number of Americans under age 25 has been stagnant this decade in all three of the Census' projected scenarios, according to Frey's analysis.
• The population under age 5 declined in all three scenarios.

Between the lines: The U.S. population hasn't aged as much as other nations like Japan — which has seen its overall population decline — largely because of strong immigration. But now immigration has been declining as well.

• Census data from last year found that the immigrant population over the last decade has grown less than in any decade since the 1970s.
• Even so, immigration has been projected to become the primary driver of population growth in this decade, given the low birth rates and the expected increase in death rates as the Boomer generation ages. It has already prevented population decline in some cities.

What to watch: The new Census data does not fully account for the near shutdown of the immigration system or the deaths caused by the coronavirus this year.

• "We're not really seeing in this yet the true impact of COVID on population change," Frey said. "If you had to do this for another year afterwards, we would see probably even lower annual growth."

About the data: The population analysis by the Census Bureau is done every 10 years and used to compare to the official Census count, which we will get sometime early this year.