Senior citizens. Could be used affectionately, humorously, derisively, or simply as a matter of fact depending on the speaker's tone of voice and whether s/he is a senior citizen.
John, give the Geritol crowd a break when you get on the bus. Those "Priority Seating" signs are there for a reason. It's very difficult for a senior citizen who needs to get around to do, say, shopping, to sit down or get up if s/he has sciatica and is forced to sit in the back of the bus.