As dogs age, they have different needs. Senior dogs do better on food made especially for them. Older dogs expend much less energy than younger dogs, so senior dog food usually has less calories and fat than other dog foods.
Senior dog supplements provide additional nutritional support they most likely are not getting from food. The most popular senior dog supplements are for joint care. The older dogs get, the less cartilage they can produce on their own. Glucosamine and Chondroitin supplements can be very beneficial to senior dogs.
Some natural senior dog food such as Breeders Choice Active Care is loaded with Glucosamine and Chondroitin. Many dog owners have wonderful success helping dog joint problems with this dry senior dog food.