questions to ask here, where was the deer killed???? genetics plays a huge part. Did you weigh the deer?? and were you present at time of kill???? If the deer is past really 2 1/2 yrs with that rack, he either has poor genetics or like woody stated, poor nutrition. How do you judge what he has??? You look at your other heard bucks, do you have a lot of main frame 8's with moderate spreads? or 10's or do you have a lot of screwed up horned deer?? On our property, we do shoot cull deer, but they are deer that members see more than once, because getting caught up in the moment of shooting a deer, emotion can over come good thought sometime. But and we try to shoot the cull deer if he is over 150 lbs because on our club, that is the average weight for a 3 1/2 yr old buck, and we pump the nutrition to them all year long. So, if he is 3 1/2 with screwed up horns = dead. None the less, it is AWESOME that your son killed the deer at 9 yrs old. I send congrats to him! Applying an age to the deer is tough because in my opinion, there are a lot of factors that contribute than just the jaw bone. For instance in tooth wear, if you are hunting in a more sandy enviroment, the deer will have more wear on the teeth ect. I hope this helps, and if you give me the info then maybe we can get close.
wes