I started out somewhere between the ages of 8 and 11 with a single barrel break action 16 gauge, I think on the grounds that it was light in weight for tramping around all day. 16 is a great gauge but the thing was though that the light weight gave it a kick like a mule and I really didn't like shooting it, certainly didn't much do it for fun. Later I downscaled to a .410 for a while but that really is a crummy, ineffective gauge to use for hunting and anyway they tell me the shells are just not around these days. I'd say a 20 or a 28 G are good for kids, but again you get little if any choice of shells for a 28, so I'd go with a 20 or all the way with a 12 depending on the size of kid. As has been stated, here in Canada 12 G will beat all the others for availability of ammo. I guess the important thing is what the kids says about shooting a 12, give him some imput.