Measure from the front of the nosepad to the front corner of the eye. If you come up with anything at or beyond 7 or 7 1/2 inches, you most assuredly have a buck that is 2.5 years of age or more. It is best to use calipers to get this measurement, for accuracy. I say 2.5 on that deer, just by looking at his muzzle. Most (there are exceptions!) 1.5 year old bucks will have an eye to nose measurement of 6 1/4 to 6 3/4 inches. Hope this helps. You do need to take a good accurate measurement though. Just measure in a straight line.

Nice kill. Congrats.