More white Americans are dying than being born for the first time in modern history—suggesting minorities and newcomers will play an important role in fueling the population growth America's recovering economy needs to thrive.

The number of non-Hispanic white Americans who died in the year ended June 2012 exceeded the number who were born during that period by about 12,400, the first "natural decrease" for this group. That's according to a U.S. Census Bureau report released Thursday and an analysis of separate data from the National Center for Health Statistics by demographer Kenneth Johnson at the University of New Hampshire. The white population edged up in absolute terms last year thanks to immigration, Census figures show.

The Census data show that as the non-Hispanic white population grows at a slower pace, the share of young Americans who are minorities is increasing, said demographer William Frey of the Brookings Institution, a left-leaning think tank in Washington. While most American children under 5 years old are still white—50.1%—that proportion is expected to fall because the majority of births have been minority children for two years in a row, a trend driven mostly by Hispanic and Asian births.

The growth rate of America's white population has been slowing for years, but demographers had expected white births to continue exceeding deaths for some time. Census researchers had projected the "natural decrease," or white Americans' deaths exceeding births, would begin around 2020 and the overall white population would start falling outright several years later.

"Even during the great influenza epidemic of 1919, there was no white 'natural decrease,' " said Mr. Johnson, calling the new numbers "stunning." The epidemic of 1918-19 killed more than 600,000 Americans. The big driver of the recent numbers, Mr. Johnson said, was a drop in white births, which fell about 13% last year from 2007 levels.