Wellness Ketoz The difference between your pet aging versus another persons pet is extreme. Cats, small dogs, medium dogs and large dogs all reach their senior years at different times. Cats usually live the longer than dogs. They often reach sixteen years old. Small Wellness Ketoz dogs are considered senior when they are between nine and twelve years old. Medium sized dogs are senior between the ages of nine and eleven and one half years. Large dogs are the unluckiest because they are considered old between seven and one half to ten and one half years.