Every child is different - and that extends to what they can take on in terms of gun-handling and shooting, too.
Don't think of any number as being that magical age. Instead, judge each child (even siblings) based on his or her own abilities and traits.
Work with their inborn curiosity, while working at the same time to demystify: to "take away the power of the forbidden," if you will.
What I mean by the latter is not to just let him have free access to the guns - rather, to make it so that there's not an "OMG you CAN'T TOUCH THAT!" kind of forbidden-fruit glorification of "the gun" that's so common in society these days.
We're inherently curious as humans, and children even more so. If a particular child is cautious around guns and understands their danger (in an age-appropriate manner), it can potentially mean that he/she is already processing at a high level - take advantage of that and lay out a good, safe, *_fun_* path for this child, and let their curiosity and abilities be your mile-markers.
My daughter got bitten by the bug rather early, so I simply let her know that any time she wanted to see my guns (and knives), that I would make it a priority to do so, in a *_
right then and there_* kind of way (not "Sure, honey, we'll do that later" and it never materialized kind of way). This not only meant to her that she was never "denied access" (although, in reality, she of course was
see what I did, there?), but that she would get my absolute undivided attention for that time, too.
With knives, I started her on my training knives. With guns, I started her on my airsoft replicas. We moved on to live baldes and actual firearms (dry) only when she expressed that she was ready. Similarly, we waited for her to be ready for live-fire before doing so with the airsoft, and yet again with firearms.
I can no longer remember when she started: I can only see how tiny her hands were - I remember laughing at how small they were, wrapped around a tiny Microtech Mini-UDT.
She was a little bit older - 5 and 1/2 - when she drilled a 6mm hole into an empty RedBull can at 10 yards, with me supporting my Tanio Koba 10/22 GBB replica on my shoulder and her taking the trigger and working the sight.
She let me know when she turned 8 that she thought she'd finally be ready to "shoot for real" by the time school let out that summer, so I made it my project to update eye/earpro and buy .22LR handguns and rifles for both of us for that season....and we've been shooting together ever since. While my M4V5LW is still a bit nose-heavy for her (she's now almost 14 and a rower - even though she's strong, the weight distribution of the V5LW as I have it configured still makes it awkward for her to manipulate off-hand), she loves the BCM/UCWRG we put together ourselves (
https://www.thearmorylife.com/forum/threads/lets-see-your-ar-setups.258/page-4#post-4362) .
This picture -
https://www.thearmorylife.com/forum/threads/lets-see-your-ar-setups.258/page-4#post-4361 - was from August of 2017 - she was 11 and 1/2 at the time. That's the ubiquitous S&W M&P 15-22 in her hands. She's got on those sports "compression sleeves" as protection from spent brass (she's still not fond of anything hot, even now) - remember what I said about making sure that the kiddo is the one having fun? Yup, cater the sessions to their unique needs.
Keep it fun, always, but ingrain safety *_now_*. Make recitation of The Four Rules something that you do together before each range day. Really hammer home muzzle awareness and trigger discipline, and give the young shooter the primacy of a medical action-plan and basic first-aid (my daughter and I high-five for a successful, "no-TQ needed day" after we pack up). Do all this
NOW so that it's simply something that the budding shooter will *always* follow and becomes second-nature, as it will help them years from now, no matter if they simply come to enjoy being a recreational shooter or develop into a professional gunman/woman.
President Ronald Regan wrote/said that "Freedom is never more than one generation away from extinction." We owe it to our children to pass on the American way of life - of freedom and liberty for all.