While it's not possible to determine precisely how old a dog is, it is possible to make a fairly accurate estimate of its age by examining the condition of the dog's teeth. In addition, a veterinarian may be able to provide an even more accurate guess by means of a physical exam or running a series of tests to look at the dog's joints, bones, muscles and organs.
As a dog ages, the condition of its teeth deteriorates at a fairly regular pace, allowing a rough estimate of the dog's age, give or take a few years. The overall health of the animal and any prior dental care also affects the teeth, which is why it's only possible to make a rough estimation.
When a puppy is about 8 weeks old, it should have all of its baby teeth, while the permanent teeth should be fully grown in by about 7 months. At this point, the new permanent teeth should be very white and shiny. At about 1 or 2 years old, the teeth have started to dull, and some of the back teeth may have started to yellow. When a dog is between 3 and 5, all of the teeth should have noticeable wear and some tartar build-up.
Between the ages of 5 and 10, a dog's teeth continue to wear down further and often signs of gum or tooth disease is present. Finally, after the age of 10, a dog's teeth should be much more worn down, with very thick tartar. Often dogs older than 10 also have some teeth missing.