Hard to say for sure, but the first one I can remember was back when I was 6 or 7 years old.
My father had an FFL back then, and was always getting cool stuff from the UPS man. I remember one day my daddy called me into the kitchen. He showed me a pair of handguns, and gave me the basic '4 rules' safety speech, and made sure I knew they were REAL guns, and that they were SERIOUS business.
He loaded the magazines (no round in the chamber), flipped the safeties on, and had me 'help' him carry the gun downstairs to the safe. The gun I got to carry was just a small .22 pocket pistol, but to me it was heavy.
I gained an important respect for firearms that day, and thanks to my fathers artful demystification of them, I largely ignored the guns until I was in High School. It was then that I joined the local '4-H' club, and began participating in their shooting sports program - on the firing line with an old single shot, bolt action, 22 was the first time I ever fired a "real" firearm. I was hooked for life.