The most reliable way of guessing a puppy's age is by looking at its teeth, according to the Humane Society Veterinary Medical Association. Young puppies are born with no teeth, but will have about 28 baby teeth when they are about 45 days old, says veterinarian Dr. Karin Szust of the University of Buenos Aires. Permanent, adult teeth replace the temporary baby teeth gradually, and most puppies have a full set of adult teeth by 6 or 7 months of age.
Instructions
Open your puppy's mouth to reveal its teeth by gently pulling the bottom lip down and the top lip up.
Look at your puppy's teeth. If you see 28 small, white, pointy teeth, the puppy is likely between 2 and 3 months old.
Check for incisors, canines, premolars and molars. The presence of permanent incisors suggests the puppy is between 4 and 5 months old. Permanent canines, premolars and molars grow when the puppy is 5 to 7 months old.
Look at the color of your puppy's teeth. In a dog's first year, its teeth are clean and white. During the second year, the dog's back teeth will start to develop a tartar build-up and look dull and slightly yellow. Between 3 and 5 years of age, the dog will begin to show tartar build-up on the rest of his teeth. At 5 to 10 years, the dog's teeth may show wear and possibly disease. Toward the end of the dog's life, it may have some missing teeth, and the remaining teeth show heavy wear and tartar build-up.