A common reason why older dogs can show aggression is pain. Pain can occur because of chronic conditions like arthritis; illnesses or injury; surgeries (for instance, the removal of a cancerous mass, not uncommon in older pets); dental disease or abcesses (causing pain when eating or with handling around the mouth); and many other medical conditions.
A dog who is experiencing pain may be sensitive to being handled. Even being gently and lovingly handled can lead to pain, causing the dog to react. Some dogs may also react pre-emptively to try to prevent the handling from occuring at all, fearing the pain that can result.
Signs that your dog may be in pain include:
Another common reason for aggression in older dogs is the deterioration of their hearing and sight. Poor hearing and sight can lead them to be more easily startled. Combined with other aging issues such as decreased mobility (preventing the dog from being able to 'escape' from annoying situations), and canine cognitive dysfunction (dementia), can all factor into aggressive behavior.
Some conditions, such as a brain tumor, can cause changes in behavior including increased aggression. It's important to have your vet do a thorough exam to check for underlying health problems. Additional tests like bloodwork or xrays may be recommended as well.
Pain, fear, anxiety, stress, and health issues can all lead to aggressive behavior in senior dogs. Ultimately, it is up to us as pet owners to talk to our veterinarians to see what can be done to help our pets feel better. Dog behavior problems in senior pets aren't always as they seem - what might appear to be an aggression issue could be a medical one instead. Old age doesn't mean that dogs automatically have to suffer - so many things can be done now to help senior dogs continue to lead happy and comfortable lives.