Aging in pets is a gradual process. Giant breeds tend to age early, for their life expectancy is generally less than 10 years. Large and medium-sized breeds have a life expectancy of 11-14 years, and small breeds can live 15 years or more. A strong, healthy dog will probably age later than a dog that is stressed by disease or environment early in his life. Dogs that are spayed or neutered before six months of age ordinarily live longer than dogs that are kept intact.