In many, many cases the answer is none. As a result, many cruise passengers with a physical disability find themselves confined to the ship for the duration of the cruise, and that's not what they had in mind. But age has nothing to do with a ship's seaworthiness, cleanliness, or style.
Some older ships are preferable to many newer ones.
It's all about how much money you want to spend, the center of gravity, and your propensity for getting seasick. On most ships, the higher the cabin the more expensive it is. However, the higher the cabin the more it moves around in rough seas. Also, forget the brochures and reruns of The Love Boat. It's where you go to sleep and take a shower.