Just like people, each dog ages differently. In general, mixed breeds and smaller dogs tend to live longer than purebreds or larger dogs. Dogs less than 20 pounds may not seem to show any signs of age until they’re around 12, 50-pound dogs around 10, while larger dogs begin to show their age between 7 to 9 years old. As a general rule, a dog who is 7 years or older should be considered middle to senior aged.