A dog's lifespan is often a matter of size. This has much to do with the amount of strain a dog's weight puts on vital organs like the liver, heart and lungs. Unlike larger dogs, small breeds simply don't carry the kind of weight that forces these organs to work harder and, in turn, burn out faster. That said, a dog's "middle age" also depends largely on its size and associated lifespan.
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Between 1 and 20 Pounds
Middle age for healthy dogs who weigh 1 to 20 pounds is usually reached between the ages of 6 and 8 (40 to 48 in human years). Dogs that meet these criteria include the Basenji, Chihuahua, Dachshund, Maltese and other toy breeds.
Between 20 and 50 Pounds
Healthy dogs between 20 and 50 pounds also tend to reach middle age between ages 6 and 8 (42 to 51 in human years). Cocker spaniels, American Staffordshire terriers, Welsh Corgis and standard Schnauzers all meet these criteria.
Between 50 and 90 Pounds
Heavier dogs weighing between 50 and 90 pounds usually reach middle age (if healthy) around age 5 to 7 (38 to 50 in human years). This applies to dogs such as boxers, collies, golden and Labrador retrievers and German shepherds.
Over 90 pounds
Extra large or "giant" breeds of dogs live the least amount of time among all dog breeds, even if they're healthy. Middle age for dogs that weigh more than 90 pounds is usually reached at about age 5 or 6 (41 to 49 in human years). Great Danes, Saint Bernards, great Pyrenees, and all mastiff varieties rarely live more than 11 years.
By Ruth Nix
DogChannel.com: Dog Size and Life Expectancy
Pet Health Network: How old is your dog in people years?
American Kennel Club: Breeds: Breed Matters
ASPCA: Pet Care: Aging
NPR: Fresh Air: Helping Your 'Good Old Dog' Navigate Aging
About the Author
Ruth Nix began her career teaching a variety of writing classes at the University of Florida. She also worked as a columnist and editorial fellow for "Esquire" magazine. In 2012, Nix was featured in the annual "Best New Poets" anthology and received the Calvin A. VanderWerf Award for excellence in teaching from the University of Florida.