Coat: As with people, older dogs may start to show gray hair. In dogs this usually appears first on their muzzle and around their eyes. Their hair may also become thinner and duller – however it’s important to see your veterinarian to ensure the thinness and dullness isn’t a sign of a nutritional deficiency.

Skin: The skin of an older dog may become thinner and more subject to injury. Dry skin can also be a problem for older dogs.
Incontinence: Older dogs sometimes have a problem controlling urination – even if they’ve been house-trained for years. If your dog starts urinating in inappropriate places it could be due to a health problem, and you should consult your vet.

Less activity: Arthritis may develop in your dog’s joints, which may make him less active if he finds it painful to move. You should still exercise your dog to keep his joints moving, but make sure you don’t overdo it.

Weight gain: Some dogs gain weight as they grow older. A weight gain may be the result of not getting as much exercise. Or, your dog may be getting too many calories in his diet. Senior dogs often need less calories, and your vet can best determine if this is the case.

Coughing: This is more common among smaller breeds. Their aging lungs may be producing too much mucous, which can block their smaller airways. Consult your veterinarian if you notice your dog coughing a lot – he or she can prescribe medication to help this condition and make your dog more comfortable.

Senses deteriorate: Your dog’s senses – hearing, sight, smell, and sense of direction – may dull as he ages. If your dog is no longer obeying you, for example, it may be that he can’t hear you. Your senior dog may also develop eye problems.

Nails: They may become brittle as your dog ages. As well, since your senior dog’s activity has likely decreased his nails may require more regular trimming since they’re not getting the same amount of exercise that walks on asphalt and other "natural nail trimmers" may have provided.

Behavioral changes: For example, your dog may back away when you pet him, or may show aggression. This may be a result of pain and discomfort.

Bad breath: This is often due to gum disease. Regular dental care, including brushing your dog’s teeth and providing him with dental checkups, and possibly a professional cleaning, can help.