The old rule of thumb in figuring out how old your dog is has always been to simply multiply by 7. So your three-year-old pooch is ready to hit the casino in human years at age 21.

Unfortunately, this doesn't fully explain how pets age.

In fact, a dog's aging process is accelerated in the early years of maturation then it slows down after that. For example, after one year a dog is actually considered to be 15 in human years.

Furthermore, the breed of dog plays a factor in lifespan. Smaller dogs tend to live longer than medium-sized dogs, who in turn live longer than larger dogs.

“Although smaller dogs tend to live longer than larger dogs, they may mature more quickly in the first few years of life,” says the Lawrence Vet Hospital in Lawrence, Kansas.“A large dog may mature more slowly at first but already be considered elderly at age 5. Small and toy breeds don’t become ‘seniors’ until around age 10. Medium-size breeds are somewhere in the middle in terms of maturation and lifespan.”

The Pet Health Network below courtesy of Fred L. Metzger, DVM, DABVP, offers a handy guide to figuring your dog's "human age".

Chart courtesy of Fred L. Metzger, DVM, DABVP