Everything I've seen up to now says that kids learn to tell the difference between right and wrong at around age 6. I mean really start to understand the difference and reasoning out why it's wrong, not just "Daddy doesn't like it when I pull the dog's tail".
I would say start teaching kids about firearm safety at around age 8. By that point, they should be capable of understanding why real guns hurt people. As morbid as it is to say, it would almost be best if they started learning about firearm safety soon after a death in the family. They would have an understanding of death and how final it is.
I grew up around toy guns and played war all the time. I also had some friends with air rifles and their parents had hunting rifles. At around 8 I started learning about firearm safety. I was not allowed to shoot a gun for almost 2 years after that, but I knew enough not to pull the trigger, to check the safety and tell a responsible adult about an unattended rifle lying around.
Then again, what do I know about kids. I'm the youngest in my family and I don't have any of my own.