A dog's lifespan is often a matter of size -- the larger the dog, the shorter its life. 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.
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.
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.
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.
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.
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