From the Census Bureau:
A meeting with the National Council of Senior Citizens resulted in President John F. Kennedy designating May 1963 as Senior Citizens Month, encouraging the nation to pay tribute to older people across the country. In 1980, President Jimmy Carter's proclamation changed the name to Older Americans Month, a time to celebrate those 65 and older through ceremonies, events and public recognition.
The number of people who were 65 and older in the United States on July 1, 2011, up from 40.3 million on April 1, 2010 (Census Day). In 2011, this group accounted for 13.3 percent of the total population.