My son is now 7 and he's been taught very basic safety since he could understand what a gun was.  We did the Eddie the Eagle cartoons regularly for a while.  I taught him some basics with a BBgun and eventually a .22.  We worked on pistol basics with a toy gun for a while and he hasn't asked to shoot a pistol yet...  but like to watch me at the range.  
interestingly he's seen the gun on my hip since he was a baby and even when we were rolling on the floor together he bumped his head on it sometimes- but never acknowledged it until the day I told him if he ever wanted to see it, he only need ask.  (after reading this article:  
https://www.corneredcat.com/article/kid ... curiosity/ )  He took me up on it immediately!  And did 3-4 times in the next hour and again for about 4 days.  Now it's old news and he rarely wants a look... but he's likes to gun shop with me and is wanting to reload but the CCW guns are "old news"  He did ask one dayif I lost my gun since why I wasn't wearing it!   

  It was because had a school meeting to go to.  SO I explained the rules of the school and guns....  he said that was "not a smart idea by the school - who would protect them if a bad guy came to school?"  Yep, at age 7 he get more than a big percentage of the adults in this country!
I think you introduce them early and then base your plan on how enthusiastic they are about it.  
An excerpt from the linked article:
........And now this same kid wanted to “just see” what would happen if he asked to see my gun.
So I handed it to him.
Oh, I checked and double checked that it was unloaded first. I made sure he knew that the gun had to stay pointed safely toward the brick fireplace while he handled it. And I stayed right there with him while he satisfied his curiosity for what seemed like the four hundred and twenty-seventh time that week. But yes, I did hand a real firearm to my energetic and inquisitive preschooler.
A few minutes later he said, “Thanks,” and handed the gun back to me — carefully keeping it pointed toward the fireplace, and not toward either one of us.....