As your dog ages, many predictable changes will occur. His metabolism will slow down and his energy level will decrease. He may spend more time in his crate sleeping. He will also gain weight so you may want to adjust his diet to one that has more fiber but less fat and fewer calories. His heart, lung and organs may function less efficiently and may become more vulnerable to disease. He may also develop arthritis and seem a little more stiff.  Pet medication such as anti-inflamatories  can do wonders for the pain.  If you keep a close eye on your dog and take him for regular visits to the vet,  chances are he will ease into a happy and healthy old age.