Estimating Age by Observing Teeth
Puppies are born without teeth but baby canines (the longer teeth on each side in the front of the mouth) begin erupting at around 3 to 4 weeks of age, with incisors (the tiny teeth in the front) and premolars (larger side teeth) coming in at around 4 to 6 weeks of age. Puppies should have a total of 28 baby teeth by the age of 8 weeks.
Around the age of 4 to 5 months, puppies will begin shedding their baby teeth and by the age of 7 months all permanent incisors, canines, premolars and molars should be in. Adult dogs of most breeds have 42 teeth.