I forget exactly how old I was, but my dad's father took me shooting for the first time with his old Remington 511 Scoremaster .22. Pap really impressed me, too, when he casually threw the rifle to his shoulder and hit targets offhand that I had missed with ten carefully aimed shots.
After that, I shot occasionally with him, and once with my uncle, who helped introduce me to big-bore revolvers. I think I was an early teen, and decided that .357 and .44 were a hoot! - but that I wanted to wait a few years before shooting them again!

I now love big-bore magnums, and load my own ammo for them.
My true marksmanship training was in high school, under the tutelage of the school's JROTC instructor, in the air rifle team. I never was the best shot, and while I scored "expert" with the air rifle, I was one of the worse shots on the team. I remain "passably competent," but am no crack shot. I DO have an affinity with most firearms, where I can become passably competent with them in a short amount of time (except Glocks, which don't get along with me), but I've yet to achieve anything approaching true expert marksmanship with any firearm.