Think simplicity, 1 rd at a time, low recoil
inexpensive. I would advise starting with something in soft air then a Red Ryder BB gun or the like. Then, when they have the concepts and safety rules down, a low-power air pistol. Then a long gun that is break action .22/.410, single shot bolt .22, Then once competent with smaller long-guns, SA .22. Work up from there, slowly and carefully. You will want to always stress safe gun handling above all else. Another place to start these days is in virtual reality. You can get video games that simulate real life situations, role play, etc. There are plastic guns that connect to computers as well as trainers that do not.
Ages are variable, depending on the child. There are certain developmental stages that prevent children from "getting it" though. A 2 Y.O. will not be a good candidate for live-fire training. LOL I start with safety from toddler age. I think you can start with plastic at 4. I would wait until 8 to give an air gun. I would wait until 10-12 for a .22 single shot rifle. 13-14 for .22 handguns (where legal) Oddly enough, it seems to me that girls are better and safer at a younger age than boys.
Long-term training in safety from day 1, role playing, testing and supervision are very important. I cannot stress enough how important early education and supervision is. Even if parents are anti-gun they should teach the basics of safety so the child knows what guns are, what to do if they find one, how do avoid them or playmates that have them etc. Look at the NRA and Eddie Eagle as one example.
You should read up on developmental psychology and physiology so you know what you might expect and when.
My .02
Shooter429