According to a recent study, 80% of dogs are classified as geriatric at age 5 due to tissue and organ degeneration. In human terms they are only 35! Dogs should have only just fully matured at this age. This is not the normal life expectancy in dogs. We have been lead to believe that giant breeds have short life spans, medium-sized dogs can live until 10 and that small dogs have a chance of reaching their teens. Back in the 1960's dogs were walking around in good shape that were 15 to 17 years old. Pet owners were feeding a homemade diet and vaccines were used sparingly. This made a difference between living to 7 years or 14 years.
Studies on laboratory animals have shown that slightly underfeeding an older animal brings back many of its youthful behaviors, and results in a substantially longer life than that of siblings given more to eat. Cut down the amount you feed your dog. You want to be able to easily feel his ribs. Regular exercise and keeping the weight off those old bones helps to keep him mobile.
Adding digestive enzymes to his food lets his aging system break down and better utilize the nutrients in his food. I give my dog 'Plant Enzymes & Probiotics' from Animal Essentials www.animalessentials.com.