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.