I’ve done 3 AQ river cruises and as others have said it’s all about the ports. From my experience, the demographics were mostly over 70 with a few younger people sprinkled in, one time even a family with two young boys! Depending on the ship, some do habe a very small pool and a small gym. Lots of people walk the decks and I enjoyed just getting off the ship at the small towns and wandering. Meeting others was very easy as the ships are small. And yes, you will be paired with others to make a larger table. I was at one that seated 4 but I was solo so we found two more that were seated at a larger table and we joined them for more fun conversation!
It is nothing like cruising on a large ocean going ship, but there was always good music, someone to play cards with, and fun entertainment at night. If you want to be active, I would suggest going on an ocean liner instead of a small river ship.
elaine