Okay looking through the American Factfinder the following metros look to have an over-65 population of over 20%. (I may have missed a few metros with that high a percent though)
Cape Coral-Fort Myers, Florida Metro
Deltona-Daytona Beach-Ormond Beach, Florida Metro
North Port-Bradenton-Sarasota, Florida Metro (One of, if not the, highest as I recall)
Palm Bay-Melbourne-Titusville, Florida Metro
Outside Florida the following looked fairly high, 15.5%-19.9%, for a metro.
Buffalo-Niagara Falls, New York
Pittsburgh, Pennsylvania
Scranton, Pennsylvania
Tucson, Arizona
Youngstown-Warren-Boardman
American FactFinder - Results