Although the total population of the United States will grow at a relatively slow rate, the age structure will be dramatically different from anything we have experienced before. While the age groups under 35 in 2010 and under 65 in 2030 will have virtually stopped growing, the number of persons 65 and over will increase sharply beginning in 2010. This Brief describes the age structure of the U.S. population in the 21st century, focusing on the elderly. The numbers used here are the Census Bureau's middle-series projections of the population. They depend on assumptions about future levels of fertility, mortality, and net immigration which imply no extreme changes in any of these components.