The short answer... as early and often as possible.
The long answer make it a way of life, not just an activity.
As many have pointed out, hunting isn't just about the kill, but all the preparation, learning experiences, practice, and mindset that goes into what we as hunters hope consummates in a kill. I like to think I started my kids preparation for hunting when they were infants as I was lying in my recliner hunting with Jackie Bushman and Bill Jordan. I made watching hunting shows a special time with me and my children, preparing special snacks, always taking the time to educate them about what we were watching and why certain things were being done.
Those lessons continued into the great outdoors at the earliest of ages, taking hikes, checking game cams, scouting new hunting spots, taking the time always to explain why we do what we are doing. Squirrel hunting was among their earliest hunts.
As far as the sitting part, I started taking them in a ground blind with me at about age 3. I took books, paper an pencil, stuffed animals, toys, and most importantly snacks! Cookies and peanuts were required hunting gear and the blind allowed them the freedom of movement. They got to dress head to foot in camo, take their own grunt call, their own pair of binos, and we would go on short hunts in the evening, in spots were even if shots wouldn't present themselves they would at least have a very good chance of seeing deer even if they were hundreds of yards away.
When I made a kill, the kids always came to see what I had taken. They took pictures with my kills, watched field dressing and butchering, and knew from the eariliest ages exactly where their food came from.
My son took Hunters Ed at 7, killed a squirrel the next day, and his first deer the following year. My daughter waited till she was 10. I let them decide when they were ready to kill their own meat, but they had been preparing as hunters their entire lives.
It's hard to beat a smile like this...
Unless it's with a smile like this!
Live to Hunt, Hunt to Live.