My son (now 19) started learning gun safety as a 1st grader through Cub Scouts. At that age(6-10 1/2) they are allowed to shoot BB's and archery. The Boy Scouts(10 1/2-17) are allowed to shoot archery, .22, shotguns, and light load muzzleloaders. Venturers(14-20 boys and girls) are allowed to add in high powered rifles and pistols.
The Boy Scouts require that all shooting activities take place under the supervision of a certified instructor. The Scouts have their own certification for the Cub Scout instructors, but for the Boy Scouts and Venturers the fire arm shooting is required to have NRA Certified Instructors and a Certified Range Safety Officer; and the archery is a level 1 USA Archery Instructor.
There is a lot more information that is required before Scouts are allowed to shoot, ie: where to shoot, proper targets, and instructor to shooter ratios, but they have done a fair job of setting up the progression from one type of firearm to the next. As one of those instructors I believe the level of safety is second to none.
Mossberg, 100 ATR, 30-06 Springfield; Ruger, P-95, 9mm Luger; Jimenez Arms, J.A. 380, .380 auto; Maverick, Model 88, 12 gauge; Beretta, PX-4 Storm, sub-compact, 9mm Luger; Colt, Lawman MK iii, .357 Magnum CTG