No one dog breed has a monopoly on longevity, but small dogs generally live longer than larger ones. Mixed-breed dogs are likelier to be free of genetic disorders that plague some pure breeds, giving them a better likelihood of living out a happy, healthy lifespan.
The average canine lifespan is based on a medium-sized dog (about 50 pounds), which can be expected to live about 12 years. Large breeds such as the Irish Wolfhound and Great Dane have shorter lifespans and may be considered elderly at only six to eight years old, while small breeds such as the miniature poodle often live into their middle or late teens. Body weight appears to be the key: the heavier the dog, the shorter the lifespan.