Like ZG said we started our daughter at 9. I myself was a very late starter since my parents didn't own firrarms. My first firearm was my Grandpas Remington 721 that I inherited after he passed, I was 29.
I started my daughter at 7 with a Red Ryder to learn safety and the basics.
The following year I bought hear a Davey Crickett and she shot CB's. Little kids I found are "noise sensitive". Later we moved up to normal .22 loads.
She's 11 now and shoots my AR's at the range. When she gets the size, I'll buy her her own AR. She can 'handle' firearms now, she just doesn't have the physical size yet.