Population pyramids show the general shape of a population's age structure.
There are many developing countries in which the age structure does resemble a pyramid, with a broad base of younger people, and few persons surviving to old age. But in developed countries, declining fertility rates have more often resulted in age patterns like the ones you see pictured here. In 1999, the United States age structure resembles a bowling pin with a slight bulge in the middle. Each year the U.S. population gets older, primarily because of the aging of the baby boomers, but also because of low fertility rates and annual increases in life expectancy . Over the next three decades, most baby-boomers will reach retirement age, causing rapid growth of the population over age 65.