There are two ways that can help us to estimate the age of a dog. 

A) Teeth
Generally speaking, the degree of teeth growth helps determine how old a puppy is whereas the degree of wear and tartar helps estimate the age of an adult dog.

Nonetheless, there are still individual differences among dogs. Plus the amount of dental care that the dog received will also affect the appearance and health of the teeth.

By and large, below are the growth stages of most canines’ teeth (if no dental maintenance is done):

 

•  By 8 weeks: All baby teeth are in.

•  By 7 months: All permanent teeth are in. They are white and clean.

•  By 1–2 years: Teeth are duller and the back teeth may begin to show some yellowing.

•  By 3–5 years: All teeth may have tartar build-up and some decay.

•  By 5–10 years: Teeth show more tartar and signs of gum disease.

•  By 10–15 years: Teeth are worn, heavy tartar build-up is evident and some teeth are missing.

 

B) Vet examination
Most vets can also estimate a dog's age based on a complete physical examination of its bones, joints, muscles and internal organs.

In older dogs, the signs of aging may be reflected in a variety of ways:

•  A cloudy appearance in the eyes

•  Graying fur coat, especially around the muzzle at first, and spreading to other areas of the face, head and body

•  Less skin elasticity - This shows up primarily under the chest, neck and around the face.

•  Stiffness in getting up, walking or running

•  Gets tired after a short distance of walk

•  Sleeping most of the time