Probably the best method is cementum annuli aging done by a lab. You pull the lower incisor teeth and send them to them and they look at them under a microscope at the lab and count the rings. Costs about $25 with shipping. www.deerage.com does it.

The next best method is to remove the lower jawbone and look at the toothwear. This is really accurate to 2.5 years old and then gets less and less accurate the older you get. Generally you can get a pretty good idea by looking at a jawbone though.

Here's a good article on the subject - http://www.huntingmuckalee.com/agingdeer.html