Hi I looked this up for you, and this is what I found
vets will first examine the teeth, if the teeth are snow white and clean , then the pup is 1 year in age or less, if the back teeth are yellow with a little bit of tartar starting to develop, then the dog is 1 to 2 years old, if the teeth have tartar build up, are yellow or brown in colour, and have wear and tear, the dog is 3 to 5 years old, if the dog has gum disease, more or severe wear and tear and tartar, or is missing teeth, then the dog is a 10 plus senior dog.
vets will examine the eyes and feel the coat and skin. If the dogs eyes are bright and clear, then it's a young dog, if the eyes are opaque or cloudy, it's an older dog. If the dog has a shiny soft coat and skin, then it's a young dog, but if the hair is more coarse and the fur and skin is greasy , it indicates an older dog.
Another indicator of a dogs age is his behaviour, some signs of old age are difficulty climbing stairs, or jumping up on things, frequent stretching, sleeping more often, and not playing as much, and the muscle tone of the dog, a young pup or dog will be more dense, and have more fat on its body, then an older dog, an older dog will have less muscle tone, and will seem to be skinnier and bonier.