The U.S. population is constantly changing. Large families like this one helped contribute to the U.S. population's 16 percent rise between 1920 and 1930.

Last year the United States had the lowest rate of population growth of any year since the Great Depression, according to Census figures released Tuesday.

The milestone is due largely to the aging of the population, with more deaths last year than at any time since 2000, according to William Frey, a demographer and senior fellow at the Brookings Institution.

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