My oldest began at age 9, but that was because I was deployed the majority of the time between 5-9.
My training regimen begins with lessons in the garage going over all the major parts of the different types of firearms, firearm safety, etc. Once they can name all the parts of the air rifle I have for them and what each part does (general terms), they get to hold it and we begin working on form.
Shooting begins with the air rifle into a BB trap in the garage. When they show proficiency with safety and shooting, then they graduate to .22s and the process starts over with shooting accomplished at the range.
I had planned on going from .22 to .243, but the Wthby Youth model I got tended to thump my oldest who is rather petite for her age. So, I got an AR-15 to add an intermediate step.