A slow introduction as has been already mentioned several ways several times; I think is key. Each child is an individual. We as mentors need to keep in mind they are watching our every move, and any single moment can cause great impact on the child's desire to hunt in the future.
I consistently see kids' interests (that were already interested) really start to crank up about 10 years old.
The age in MO is 6. I shutter when dads of 5 year olds start claiming aloud all the things their child is going to do in the woods the next year, especially in front of the child. I feel they are being set up for failure by the boisterous claims. (Getting 20 feet up in a treestand, sitting all day, they don't need an iPad to keep their attention....they can HUNT!, they can carry their own loaded gun to the field or they can stay home, they can only hunt during youth season and they better make the best of it, etc.)