What is the proper age to begin teaching?
When they are big enough to pick up a gun. That is when the teaching should start. When they are big enough to pick one up, they need to be taught some things. Not much at first, just a few simple things. Even though they are not big enough to use the gun yet, they can learn, and should be taught.
Unfortunately, kids start learning about guns the moment you set them down in front of the TV set. And what they learn is all wrong! They do not see responsible gun use on TV, and very little other responsible behavior either.
It is up to the parents to make up for that lack. Not teaching children the true facts about firearms is to me, child abuse.
It used to be that familys trained the children about guns, the same as they did with everything else in life. Today there are few families that still do this. Many, many children get the only firearms training they ever get from the TV, video games, and peers in the same situation.
Each stage of a child's developement has a level of firearms training that is appropriate to it. Not all children are at the same stage of maturity at a give age, so the level needs to be tailored to the individual child. But there is a proper amount of training for each.