While we don't have any cruises that specifically cater only to seniors there are certain lines and itineraries that have more mature guests onboard.
Cruise lines that typically attract a more mature audience would include Princess, Holland America, Azamara, Silversea, Seabourn Viking River, and Cunard.
Itineraries that attract a more mature audience would include Panama Canal, Asia, Transatlantic, Transpacific, and River Cruising.