In general, dogs age in a manner similar to humans. Their bodies begin to develop problems that are less common at younger ages, they are more prone to serious or fatal conditions such as cancer, stroke, etc., they become less mobile and may develop joint problems such as arthritis, and in old age often become less physically active and may even develop dementia. Additionally, they become less able to handle change, including wide climatic or temperature variation, and may develop dietary or skin problems or go deaf. In some cases incontinence may develop and breathing difficulties may appear. "Aging begins at birth, but its manifestations are not noticeable for several years. The first sign of aging is a general decrease in activity level, including a tendency to sleep longer and more soundly, a waning of enthusiasm for long walks and games of catch, and a loss of interest in the goings on in the home." see https: //en.wikipedia.org/wiki/Aging_in_dogs#Importance_of_diet_in_aging