When they are old enough to understand the difference between right and wrong and understand what the word "no" means.
Each child will be different. You have to judge when is the right time. My dad wouldn't let me touch a real gun until I was able to recite his rules of gun safety and could show proper safety with my toy guns. His house, his rules. I was about 7 or 8 when I fired my first gun and even then they were never taboo in our house. I knew that if I wanted to see one all I had to do was ask and he was more than happy to be their and supervise while I was handling them. I never went to get one on my own until I was around 12 or 13 and he said it was ok. I was pretty aware of what the consequences would have been had I done that before I was aloud.
Freedom is never given. Only taken, and paid for.......usually in blood.
"Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
President - John F. Kennedy