Immigrants and children of immigrants keep U.S. population young Washington – A steady stream of immigrants to the United States and moderate growth keep the U.S. population younger than other developed countries, according to the U.S. Census Bureau, which projects population in the United States on New Year’s Day 2007 to be 301 million people. Younger population growth keeps the economy and society vital, William Frey, a demographer at the Brookings Institution in Washington, told USINFO. “With people coming into our country from around the world, as a population overall, we have become more open to new ideas,” Frey said....