How old is that deer?
 

Art by Bob Travers

The only way to truly tell the age of a deer, is to examine the teeth. 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.
 

To determine whether a deer is 1-1/2 or 2-1/2 years old, examine the upper third molar. In yearlings, this tooth will be partially erupted through the gumline, but not all the way. The tooth will not show any signs of dentine.
 

 
In 2-1/2 year olds, the upper third molar will be fully erupted, and may already show signs of wear. The dentine is also now visible.
 

 
In 3-1/2 year olds, the first molar will show signs of wear. The dentine of the first and second molar will be wider than the enamel on the teeth.
 

 
At 5-1/2 years old, most teeth will show signs of wear. They will have small ridges and the dentine will be wider than the enamel
 

 
At 7-1/2 years old, all teeth are worn down. Most are damaged, chipped, or broken. Most teeth have worn themselves to the point of being flat, except for the last two molars.
 

 
If a deer reaches 10-1/2 years old, the teeth will be worn down to or even below the gumline. The dentine is barely visible and some premolars may be missing. The last two molars are now clearly worn and almost flat to the gumline. The front incisors may also be missing or worn to the gumline.