The most foolproof way to age deer is to build history with them. Of course, that takes time--hunting the same private farms year after year, building up trail cam history and spending time in the tree. But then there are those circumstances where you don't have that history and have to make a split-second decision on an unfamiliar buck.
One of the first clues to a buck's age will be its general body size. If he's on his own, that may be a little more difficult to judge with no other deer to compare him with. But if he does have another deer with him, typically a mature buck is going to tower over a two- or three-year-old deer and especially over a doe. Depending on the time of the year that it is, they can get substantially larger.
Other identifying features of a mature buck include a flat back, broad chest, and large belly. You can often go off the face of a buck as well. At a distance, body size may be the only indicator, but as a deer moves in closer, you might be able to utilize this relatively foolproof way of determining his age. It is not often that a buck's face will fool you. The older bucks get what we like to call that "warrior" look to them--a grayer muzzle with a darker forehead.
Once a deer reaches five and a half, six years old, they're going to have larger mass around their brow tines and throughout the rest of their entire rack too - and it is at this age that their rack normally reaches its largest mass in their lifetime.
Ultimately, developing a history on hunting properties, whether that's running trail cameras or spending time in the tree, is the surest way to familiarize yourself with a buck and get a sense of his age. However, short of that, hopefully some of these tips will help you make that split-second decision next time an unfamiliar buck is in your sights.
--Midwest Whitetail