Between 1950 and 2004, the U.S. population got older. During that time, the under 18 population fell from 31% to 25% of the total population; while the 55-64 population grew from 9% to 10%, the 65-74 population stayed at about 6%, and the 75 and older population grew from 3% to 6% of the total population.
Centers for Disease Control and Prevention, National Center for Health Statistics. 2005. 'Health, United States, 2005: With chartbook on trends in the health of Americans'.