I think you can include them in "hunting" as soon as they are home. My son who is only 16 months, his 2nd word was deer. He likes to check out my mounts and touch them, we did have 1 little accident with the bear falling on him when he rammed into it with a truck but nothing a little therapy later can't fix. He has seen and pet birds I have brought home and he's thought some of the deer hangingfrom the polewere pretty cool. He loves to go for walks with me, and the best investment any new Dad can make are those back pack things and go for walks in the woods.
He loves my turkey and predator calls and he'll be out scouting and hanging stands this year with me before he's 2. As far as when he actually goes into the field with me to take game, that will depend solely on his maturity level and interest. He's already part of my "hunting expereince" though......