I started my twins around 3, and they are now 5. They know Eddie Eagle, know to ask anytime they want to see a gun, and know where to point it when they do get to see it. Both own rifles and a shotgun, and their first handguns are already in the safe (matching Savage 101s). My daughter knows the difference between a clip and a magazine! Both can tie pressure bandages. I caught my boy climbing on top of the refrigerator at 3, so believe me if you think you can get away with hiding guns in high places and be safe you're wrong.
The twins have been tested a few times, either by me intentionally leaving an unloaded gun around, or in a real situation, and they remembered when it counted.
If you've seen the new movie "Brave", the queen keeps yelling at the princess for leaving her weapons on the dinner table. When she said it, both kids leaned over to me and said "see, Dad...no weapons on the table!"