I watched my friend play the first three dungeons one day (and when Ganondorf got the Triforce) and I thought, "I've got to get this game." So, I guess it was then I knew it was epic. (Funny, though, I only remember a few bits and pieces of it, like getting the Kokiri Sword, Jabu Jabu, the Temple of Time and Ganondorf. And a little bit of Adult Link playing the Ocarina.)
I didn't get it until a while later, actually. I don't know when I watched my friend play it, but I got the game used in '99. Same kinda thing with Majora's Mask. I had no idea about either game until I saw them. (Actually, I'd heard about Majora's Mask before I saw it. So, whatever. But I bought Oracle of Ages before MM. :xd
I also didn't have an expansion pack before I got MM. I told my cousins at some family reunion, they said they had one and they'd bring it to me, but they live in Florida, so I had to wait until Thanksgiving. ...Which was quite a few months away. It was agonizing...
Oh, yeah, thing is, a different friend showed me Majora's Mask on the GameCube Collector's Edition, so I didn't play it for quite a while after it came out. I can't remember if I already had the game when he showed me or not, though... Anyway, when I played Majora's Mask, that's probably when I truly became a Zelda fan.