If your dog is 6-years-old, that really means he’s 42-years-old in human years, right? Not exactly. For decades, a popular calculation from dog to human years has been that 1 is equivalent to 7. Based on research, it doesn’t seem that simple.
The most important factor when assuming a pet’s age equivalency to human years is size. The American Veterinary Medical Association (AVMA) states that small dogs and cats are generally considered “senior” at 7 years of age. Larger breeds tend to have shorter life spans and are often considered “senior” at 5 to 6 years of age.
The AVMA uses this general guideline (image from AVMA):
You might be asking yourself, how do researchers come up with the numbers? The American Kennel Club explains “The ‘senior’ classification is based on the fact that pets age faster than people, and veterinarians start seeing more age-related problems in these pets. Contrary to popular belief, dogs do not age at a rate of 7 human years for each year in dog years.” They also explain that the National Center for Health Statistics don’t actually keep records for dogs, but instead use three sources of data on their longevity: veterinary hospitals, pet insurance companies and breed club surveys.
Historically, people have compared human years to dog years for centuries. Evidently, in the 1260s, artisans creating the Cosmati Pavement in Westminster Abbey inscribed into the floor a prediction for Judgement Day: "If the reader wisely considers all that is laid down, he will find here the end of the primum mobile; a hedge lives for three years, add dogs and horses and men, stags and ravens, eagles, enormous whales, the world: each one following triples the years of the one before.”