From the standpoint of nutrition, your older animal is very much an individual. As an adjunct to good nutrition, consider exercise and encourage moderate activity. If your dog's health permits, walking is great exercise. Just be sure to start any exercise program gradually. For water-loving dogs, swimming is good aerobic exercise.
Swimming is especially good for dogs with arthritis because there is no jarring impact.
Even a game of fetch helps maintain blood circulation and muscle tone. Sadly, the time may come when you have to make the difficult decision about letting your beloved pet go. Your final act of kindness may be to ease the pain of your long-time friend. But until you need to contemplate such a profound decision, give your senior citizen a little extra care, understanding, and attention. Savor your the golden years.