I became a fan at age 10, in 1978. I had seen the reruns before, remembered the cartoons, and even taped the audio from one Season 3 episode, but it had been out of syndication in my area for a couple of years when I actually became a fan. Needless to say, I subsequently wore the heck out of that one audio cassette and still have it committed to memory.
It was just a chance occurrence in 1978; I bought one of the Alan Dean Foster books at a school book fair. (Log Four it was.) From there I was hooked, devouring all the Blish books, the Foster stuff, and the few Bantam novels and "New Voyages" collections that had been coming out at that time.
Quite frustrating that I wouldn't be able to see the show on TV for another couple of years. The Mandala Fotonovels, Power Records, Gold Key comics, and even Mego action figures were invaluable consolations during that time. Naturally I saw the flick and bought the soundtrack LP; subscribed to the Marvel mag. When the show finally came back on the air where I lived (albeit at 1:00 a.m. on Sunday mornings), I could at last breathe the pure air of "proper" Star Trek.