Vitamins are catalysts in chemical reactions in a living organism. When the supply of catalysts is meeting what is needed, more supplementation may be come toxic. If a lot of vitamins are supplemented and not needed, they are eliminated in the urine thus resources are wasted. Vitamins are not a "quick fix" towards healthy.
Supplementing older cats may not be a good idea. Every nutrient has toxic levels. More is not better. Dietary change to senior pet foods with appropriate dietary ingredients may all that is necessary. Always ask your veterinarian before making changes to your pet's food.