I started to learn to shoot a 22 rifle when 6 years old with tight supervision from dad when a colt hunsman later that year. by 12 i could go behing dads budiness and shoot on my own. Shot some most weekends. I was hunting with a 12ga springfield pump for deer at 13 and had a old '53 jeep pickup of dads to drive in the woods. Seems sheriffs and wildlife officers were fine with us back then .
Not sure age has as much to do with when a child first learns to shoot. More about how there brought up and how they handle the chance to learn about them and to shot them.
Like others said I know 15 & 16 years old not worrthy of handling a firearm much less shot them.