The age of a puppy can be determined by obtaining the exact birth date and counting the weeks that have passed since that date. There are other ways to determine the age of a puppy if the exact birth date is unknown, including the rate at which the teeth grow.
A vet can help determine the age of a puppy by performing a physical examination, primarily of the teeth. By eight weeks, a puppy has grown in all of his baby teeth. Around seven months, the baby teeth are replaced with permanent teeth. As the puppy ages, the teeth show signs of wear. This appears as a dulling in color and yellowing.