Traditional wisdom has said that one dog year equals seven human years. This is an oversimplified view of how old your dog is in human years. Veterinarians have now come up with a much more realistic comparison between human and dog years.

Size and breed also influence the rate at which a dog ages. Although smaller dogs tend to live longer than larger dogs, they may mature more quickly in the first few years of life. A large dog may mature more slowly at first but already be considered elderly at age five. Small and toy breeds don't become "seniors" until around age 10. Medium-sized breeds are somewhere in the middle in terms of maturation and lifespan. The below chart is based on medium-sized breeds.