No real magic number... My son was 7 when I got him the Airsoft. Age 8 when he went with me for his first range trip. Age 9 he had his own 22 rifle (legally mine until he comes of age)... He was 9 1/2 when he fired my 22 pistol, Beretta 92FS, M&P45, and AR15... He's a great shot with both the 22 rifle and pistol. He understands the power of the weapon and has respect for it. Safety is paramount and he gets that.
My daughter has fired my 22 pistol and the 22 rifle. She fired the AR15 once and has not come back yet for more. She prefers the rimfire stuff for now... small steps... You'll know when they're ready... Good luck and stay safe... Don't worry about the naysayers and the shock and awe folks, they don't get it. The passion, respect and love for the shooting sports...