The sooner the better. Granted, they need to be able to handle the weapon physically. I learned at the age of 4 or 5. I would go pheasant hunting with my dad and grandpa. I didn't use the 12 gauge at the age, but my dad always brought his semi auto 22 pistol. When the day was done he would always find something for me to shoot at with the pistol. At the ripe old age of 12, he bought me my first shotgun, a 12 gauge pump from Sears. He bought me that so he could have his 12 pump back. I was always gone with it and hunting. My opinion, when they can physically handle the recoil, hold it correctly and understand, that is the time to start. Teach them what guns are for, demonstrate what they will do, make sure they understand "once the trigger is pulled, you can't take it back", and my dad's favorite..."if you aren't going to eat it, DON'T SHOOT IT!" I must have heard that a million times. Respect for a gun goes a long way and is easy to teach. A big part of gun handling is, "Don't be an example, set one!"