There's an old myth that dogs age the equivalent of seven human years every year they are alive. While true on average, the rate of aging in dogs varies significantly between breeds. In general, small dogs reach adulthood faster than large dogs; but small dogs have a longer lifespan overall.
The First Year Of Life
Scientists are not sure why, but size matters when it comes to a dog's lifespan and how quickly he grows up. Small dogs get older quicker: a Pomeranian, Norwich terrier and toy poodle are all 15 years old in human terms when they reach the age of 12 months. Larger dogs lag behind: an Old English sheepdog, Labrador retriever and German shepherd are all considered to be 12 years old when they have been around for 12 months.
Even though small dogs age quicker, they live longer. A bulldog ages an average of 13 years for every human year while miniature dachshunds age just over four years for every human year. To further illustrate, consider that a 10-year-old English setter is considered to be 75 in human terms, while a 10-year-old beagle is considered to be only 56.