From The American Presidency Project, Proclamation 5847—National Senior Citizens Day, 1988:
"Throughout our history, older people have achieved much for our families, our communities, and our country. That remains true today, and gives us ample reason this year to reserve a special day in honor of the senior citizens who mean so much to our land.
“With improved health care and more years of productivity, older citizens are reinforcing their historical roles as leaders and as links with our patrimony and sense of purpose as individuals and as a Nation. Many older people are embarking on second careers, giving younger Americans a fine example of responsibility, resourcefulness, competence, and determination. And more than 4.5 million senior citizens are serving as volunteers in various programs and projects that benefit every sector of society. Wherever the need exists, older people are making their presence felt—for their own good and that of others.”
Source: U.S. Census Bureau, Population Division, Demographic Analysis (DA), Special Tabulation
From Census.gov > Topics > Population:
Source: 2017 National Population Projections Tables: Main Series, Projections for the United States: 2017-2060.