How young is too young to teach kids how to shoot a gun?

I know of many kids that learned at a very young age (2–4) how to shoot a gun. Surprising to me since I’m imaging the kickback from even the smallest gun could knock a kid on his rear. What do you think is the youngest age could/should be for kids to learn? When did you learn? Would you be afraid of your kids being around other kids that have/can shoot at a young age?