Dogs also suffer from age related illnesses according to researchers, who claim that they may go mad in their old age.
A survey of 1,000 owners revealed that a third of pets aged eight and older become forgetful, bad tempered or do not know where they are: all major signs of "cognitive dysfunction".
Other behavioural changes could include disrupted sleep patterns, disorientation, changes to toilet habits and even hostility.
Sarah Heath, a leading animal behaviour expert, said: "Pets, and particularly dogs, over eight years of age are more likely to start displaying signs of cognitive decline, which can result in them becoming less sociable with familiar people and animals and appearing disorientated within their own homes or out on familiar walks.
"Cats can also suffer from age-related brain changes and owners of old cats should also look out for behavioural changes in their pets.
"Although these signs can be similar to symptoms of Alzheimer's in humans, many pet owners do not often recognise it in their animals," she added.