This myth comes from the idea that the average life expectancy of medium-sized dogs is one-seventh of humans. That’s not a completely accurate unit of measurement to begin with and does not account for different dog breeds, sizes, health concerns, etc. According to veterinarians, there are six stages of life for canines. Those are puppy, junior, adult, mature, senior, and geriatric. That’s somehow sweet and soul-crushing at the same time.