Deer are born with four teeth on their lower jaw. These four front teeth are called incisors. After a few weeks, sixteen more teeth grow in, giving it eight front incisors, six premolars on the bottom jaw, and six premolars on the upper jaw.
When the deer is one year old, six more molars erupt on both the upper and lower jaws. This gives the deer a full set of 32 teeth. The darker material in the tooth is called the dentine. As the hard enamel is worn away, more dentine is visible. The amount of visible dentine is an important factor in determining the age. The tooth wear and replacement method is not 100% accurate however, due to the differences in habitat. Tooth wear on a farmland deer may not be as fast as that of a deep woods buck.
The most accurate way to tell a deer's age is by removing a tooth, cutting a cross section of it, and counting the rings under a microscope, (much like aging a tree). Each winter, when a deer's blood-serum protein and phosphate levels are low, a layer of cementum is formed on the tooth. Therefore the tooth has one layer for each winter the deer has lived through.