According to PetMD, the average life span of a mixed breed dog under 90 pounds is 11 years, while larger dogs live for about eight years. For purebred dogs, life expectancy varies by breed.
For instance, a bulldog typically lives for only six years, while miniature dachshunds and poodles have a life expectancy of 14 years. Another way to consider the life span of dogs is to compare it to human years. Again, this varies by a dog's size. At a year old, all dogs are about 15 "human years," but larger dogs age faster. At five years, a small dog is around 33 in human terms, but a dog over 90 pounds is about 42. In three more years, the small dog compares to 45 years, but a very large dog is 63.