I plan on starting my sons when they turn 5 or 6 depending on their interest level. My oldest is about to turn 4 and loves to "help" daddy clean the guns. By help I mean he sits near me on a stool and asks a million questions. He has a genuine interest already and my wife and I have taken the Eddie Eagle approach to gun safety with him and I gotta say... IT WORKS! At least with him. When I lay my guns out for cleaning if I leave the room he follows me because he knows he's not to be in the room with my guns by hisself. I am really looking forward to buying him a pellet gun in a couple of years and taking him out for target practice.
All good input.....I'm with gasmitty on this. Now all you have to figure is a plan to get them away from the TV and Video Games!! (good luck here).
I taught my friends kids about guns with the parents permission starting about four years of age. Mainly be letting them watch me clean the guns and then progressing to letting them help me clean them. I was more concerned about them handling the cleaning materials that I was about them handling the weapons themselves.
This is the way my Father introduced to firearms and it worked for me. I was allowed to unload his handgun and clean it every day when he got off work. He was a cop.