Good question. For me it happened about 50 years ago, and the very cool thing is that I can still see the scenes, smell the smells and hear the sounds as if it happened yesterday. The Marines would give regular shooting lessons to dependents and I was one of about 10 young kids shooting .22 bolt action rifles on the same firing line as Marines doing their practice. There were no benches or rests as I recall. On my left was a small group of Marines practicing with their issued 1911's, including my father. On the right about 20 yards away was a group practicing with M1's. I think I watched both of those groups, especially the 1911 shooters, more than I concentrated on my own target. I was hooked solid at that point and there was no turning back.
Does this sound familiar to any of you? " CEASE FIRE - cease fire - open bolts - unload"
That was what the range officer yelled at the end of a firing round. I can still recall the sound and the cadence of it to this day. You had better not even think about squeezing off another shot after that command was given.