As soon as they can walk and are interested. My 11 year old was started at 3. She wanted a gun like Papa. We got her a Daisey Buck that has about a 42# trigger pull. I’d help her shoot it from time to time but she got handling and saftey skills down pat. We honed her shooting skills with a cricket starting at about 5. She did 2 years in the NRA YHEC program which is genrally excellent in most areas. And now she has an MP15-22 and is also shooting a NEF 20ga slugster. Key is the maturity level and concentration of your kid and recoil and weight management for the kids. If it were not for the socialist state of MD she would have a full up rifle on the rack for her now.
Professional training as soon as possible. Training scars that affect safety that have been there since they could walk are hard to retrain in a few hours.
Start with safety and the. Contact a reputable trainer. Like MVT for example! Always ensure they have eye pro, ear pro and start out on small caliber guns one bullet at a time. In other words don’t hand a 9 year old a fucking fully loaded Uzi with a giggle switch. She might canoe your head.
We started our girls at age 3 with a single shot Cricket, with a cut-down stock. We started on paper targets, and as soon as safety was second nature, we graduated to birds and rodents. I think this is important for several reasons, actually killing things is much different than killing beer cans, living things run and hide, and of course you must determine if your target is shoot-worthy (is it a starling, or a red-winged blackbird, or is that ground squirrel too close to the tractor tire?).