I coach youngsters, we had a shooting session last weekend joined by a new boy, 13 years old, he tried my 12 ga 11-87 12 ga with target loads, and it was too much for him too heavy and the recoil was too much.
So he shot my 1100 28 ga and loved it, but we were low on 28 ga shells.
So he tried my Lanber o/u 12 ga with 20 ga Chambermates and 20 ga shells and that was fine he was hitting them really well and no bruising.
I am set up to load 28 ga, and I think I'll get some 12-28 Chambermates for the smaller kids and ladies to use.
I'm involved with the Delta Junior Waterfowl Hunt as the instructor and I just wince when I see a 12 year old with a single shot hammer 12. Delta donated to our group a Turkish semi-auto and we use that as a loaner gun.
My experience is that 12 is about as early with a shotgun, 8 years old and up, a good air rifle or a 22 single shot on cans and targets under close supervision. My 2c, anyway, 30 years of Hunter Ed and training people of all ages.
Mike