Older cats are also more prone to digestive diseases like inflammatory bowel disease or urinary tract problems, diabetes, hyperthyroidism, etc. No, as before, unless there is a specific medical reason and if a cat is generally healthy, supplements aren't necessary. One exception may be those cats that are not on a commercially available quality prepared cat food. If your cat has a home-made diet or uses a lot of hairball remedies, Vitamin D decifiency can occur. Maybe - only a very small dose is needed, and overdoing it is easy and dangerous.
Comfort is a big issue with anyone, especially as one ages.
For cats, an essential comfort is a steady routine with little environmental change. Stairs might be more difficult to climb, especially if arthritis has set in. If you have stairs, you want to be sure that water and a litter box is available both up and down stairs. Warmth seems to be very attractive to most cats - that's why the sunny window is wonderful. One blessing of older cats - they tend to be, much more, lap cats.