If for whatever reason you don't know your puppy's age, you can find out by examining his teeth. A fully grown dog has 42 permanent teeth, but as these grow at different times you can estimate your puppy's age by examining the adult teeth that have already come through.
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Growing Adult Teeth
Open your puppy's mouth by placing your hand on the bridge of his nose and sliding your thumb behind his back tooth.
Turn your flashlight on with your free hand and check how many teeth your puppy has. If he has 28 teeth small, white pointy teeth then he is between 6 weeks and 3 months old.
Look for any larger teeth among the smaller ones. These are the permanent teeth and start to come through when your puppy is around 2 to 5 months. Different teeth come through at different times. After 5 months, the central teeth, the Intermediate teeth, the corner teeth, the canines and the first premolars should have all come through. At 6 months your puppy should have his second, third and fourth premolars and his first molars. When his second and third molars come through at 7 months he will have his full set of adult teeth.
A Dog with a Full-Set of Permanent Teeth
Examine the color of your dog’s teeth. If your puppy's back teeth are just starting to turn yellow, he is between 1 and 2 years old. The older your puppy is the more yellow his teeth will be. If the puppy is between 3 to 4 years old, you may notice a dark discoloration at the top of the tooth and on the gum line. This is the start of the build up of tartar.
Seeing large amounts of tartar extending down from the gum line onto your dog’s teeth indicates that your dog is between 5 and 6 years old.
Seeing brown, blunt, worn teeth, as well as an increase in tartar, indicates that your dog is 7 years old or over.