This. Every kid is different.
My sons started shooting BBs and airguns at 5, 22s at 7 or 8, heavy rifles came shortly after. I held off on pistols until after 10. The boys pretty much determined when they were ready for the next level based on comfort and strength. They each got a box of AR parts at 14 to build their own rifle and understand how it works. All of this was under my watchful eye. I didn't let them go shooting on their own until around 16, I might not have been right at their elbow every time, but I was watching and making sure they were following my rules. Once they had proven to do it right even when they thought i wasn't watching, I started letting them go on their own.
My daughter is 29 and I would never let her shoot without me right there. She recently made mention of looking for a self defense weapon and I told her to get a baseball bat.
In my experience, for the average kiddo, start them early with shooting. It takes the mystery out of it. Bring stuff to destroy, watermelons and such. Let them learn how powerful firearms are. Let them learn a gun is a tool and not a magical pew pew like the movies. I didn't have a safe for a long time and never worried about the boys going into it without supervision. Recognize there are some children who should never be granted access to firearms.... like my daughter. :)

