The Unite States is an urbanized nation, with 80.8 percent of its population residing in cities and suburbs as of mid-year 2005. The mean population center of the United States has consistently shifted westward and southward, with California and Texas currently the most populous states. U.S. population growth is among the highest in the developed countries, although like most of them its rate, 0.88% annually, is below the world average rate of 1.16% annually. The total fertility rate in the United States estimated for 2007 is 2.09, which is roughly the replacement level for industrialized countries. Children (people under age 18) made up one-fourth of the population (24.6%), and people over age 65 one-eighth (12.4%) in 2006.