Where I grew up, you almost had to know how to use a gun. Everyone, and I do mean EVERYONE, in my area knew the basics of the pistol, the rifle, and the shotgun.
To start my Dad gave me a brand spanking new Daisy, lever action, BB gun when I was 3. We spent 4-5 weekends making sure I knew how to handle it. Then he spent a few more weekends only letting me play with it when I was with him. Then he let me play with it whenever I wanted.
Around age 4, he was letting me use a single shot .22 rifle that we had. As skill progressed, we went up to an old 16 shot, tube loaded, semi-automatic .22 rifle. At age 5, he gave me an Colt 9 shot .22 revolver, with a slightly bent barrel.
I still have all 3 to this day.
Of course, in the part of the country that I lived in, and I do mean country, this was not unusual. The nearest gas station was 30 minutes away. To go to the nearest movie was a 2.5 hour drive. The nearest hospital was just under 2 hours.
The primary reason that we learned how to handle firearms was to handle local varmints, such as wild dogs, wildcats, and snakes, as well as to hunt food. I took my first deer with a .22, because I didn't know you weren't supposed to be able to do that.
I ate more squirrel than the Goddess intended any man to, because that's all we had to eat.
Of course, we were so backwards that I still remember openning fire on a group of kids with some rat shot, because they shot at me with a pellet rifle. I still remember that their parents were only upset because I scorched their clothes.
Now, in current times, particularly if you live in the city, I don't think it's that vital to teach kids at that young an age. Unless you feel that they might come across a pack of wild dogs (which will put you in the hospital real quick...and that's personal experience talking), or need to hunt for food, I honestly can't say that I agree with it.
On the other hand, though, I took my God Children shooting for the first time at age 5, with that same single shot, bolt action, 22.