Deer build a new layer called cementum on their teeth each year, much like the annual rings of a tree. By freezing a tooth, a biologist can slice off a thin layer and determine the exact age of the deer. Hunters can estimate the age of deer they take by evaluating replacement and wear of teeth on the lower jaw. Both mule deer and whitetails can be aged by this method.
To check replacement and wear, you will need to slit the check bone to expose the teeth on the lower jaw.
Fawns have an incomplete set of teeth with three premolars and one molar on each side of the lower jaw.
A yearling (1.5 years old in the fall) will have three badly worn premolars. These are temporary teeth and will be replaced next summer. The third premolar which has three crowns is also characteristic of a yearling. When replaced, this tooth will have only two crowns.
The 2.5 year old deer has a full set of permanent teeth. These are characterized by clean, unstained new premolars and molars with sharp, pointed crowns showing little wear.
A mature deer has 20 teeth in its lower jaw. There are eight incisors, six premolars, and six molars. From here on the age of your deer is determined by the relative amount of wear on the teeth. A mature deer (3-7 years old) has teeth that are still in good condition, showing wear, but some still have pointed crowns. In the mature old class (8 years or older) the teeth will be worn almost flat, sometimes nearly to the gums.