All kids are different so there's no good answer for when to start them. Aside from the physical aspect of being able to shoot a gun there is the mental aspect of dealing with death. All my kids went in the woods with me and saw deer die before they pulled the trigger themselves.
My daughter who is now 12 started hunting when she was 9. My son is about to turn 8 next month and he started a week before he turned 6. My youngest is 5 at the moment and he'll be 30 before I let him have a gun. That boy is wide open.
The older two both started hunting with a cva wolf muzzleloader that I cut down the stock on. Small kids have a short LOP. 40 grains of pyrodex and a round ball is a fun plinking load. 60 grains of pyrodex and a 240 grain xtp has accounted for 9 deer between the two of them. All were pass throughs at distances to about 70 yards with the exception of a big bodied 6 point that was shot quartering to us, that bullet lodged behind the last rib under the hide.
They've both shot AR 15s in 223 and 6.5 grendel. Adjustable stocks help with LOP. My daughter plans to use the grendel in Virginia for rifle season this year. She's also shot a remington 700 in 243, but prefers the grendel.
