Well, like others have said, I was pretty much born one. Never turned down a chance as a kid to go shooting. That all stopped around age 9 I think though due to a split in our extended family.
As an "adult" at 21 I got an opportunity to shooting again -- I jumped on it. Ended up buying some pistols within a couple of weeks of that IIRC.
Now, when I -realized- I had become a "gun nut": I was out shooting with a buddy at his parent's house. When we got done I packed up whatever I had brought (probably a CZ-75, Glock 21, AR-15 and AK-47 clone), and the "extra" ammo we didn't shoot, which was around 200rnds left in an army surplus ammo can. I actually said to my buddy, "Ya know those guys you hear about that everybody thinks is crazy because they've got a buttload of ammo and guns all the time?" ... "Yeah." ... "I'm pretty sure I've become one of them. Oh well." We kind of laughed about it.
Few months later (didn't shoot during the very cold winter much) I was at a gunshow with the same guy, my brother, and another friend. We just sorta "stopped in" since we were coming back home from watching a wrestling meet. Weeelll.... I saw myself a Soviet M44 wrapped in paper and twine with explicit instructions on it to NOT remove the paper. I asked if they were unissued/unfired, sales person said yep... so I said I'd buy one. Brother looks at me (not a gun guy) and says, "Why are you buying THAT.. you can't even see it!"... my reply was something along the lines of "It's a nice gun... trust me." He just rolls his eyes. Solidified, right there.