Dog longevity is much more varied than human longevity. For dogs, size, physical characteristics, breeding and genetic disposition play roles in life span. While you can gauge how long dogs of a particular breed may live on average, you can't predict factors like disease that crop up unexpectedly.
Typically, smaller dogs live longer than larger dogs. According to WebMD, while almost 40 percent of small breeds, like Chihuahuas, live beyond 10 years, only about 13 percent of giant breeds like Great Danes make it to that age. A giant-breed dog is typically considered elderly once he 6 or 7 years old. Breeding also matters: Purebred dogs tend to have shorter lives than mutts, because of inevitable lineage inbreeding. Genetics and physical characteristics common to some breeds also play into life span. For instance, some dogs are more susceptible to cancer, like golden retrievers and boxers.
Tom Ryan is a freelance writer, editor and English tutor. He graduated from the University of Pittsburgh with a degree in English writing, and has also worked as an arts and entertainment reporter with "The Pitt News" and a public relations and advertising copywriter with the Carnegie Library of Pittsburgh.