Started teaching my grandson when he was about 3. Started with a Red Ryder and moved him up from there. This December, at the ripe old age of 4 3/4 he took his first deer. His Dad was there in the stand with him and other than putting the rifle up on the sand bags, B did the rest. He found his sight picture, took his breath and squeezed.
Seadog has hit the nail on the head. Each kid will be different. I took my first one the week after I turned 6. I'd been shooting for about 2 years by that time and Dad thought I was ready. I was.
Kid's attention spans at those young ages is pretty short. Range time with them seems to be more time spent setting up and taking down than actually shooting. Keep it fun and educational and they'll want to go back.
It's always great when you have a 'new' hunting buddy!