I have memories of watching the show as far back as 1993 or so, but it was never intentional - when I was a kid, my mom told me that I wasn't allowed to watch the show until I was 13, and I seriously took that to heart, going so far as to bolt from the room in terror whenever I heard the opening theme song beginning (I was genuinely afraid of the show for a while, since my mom never told me why I couldn't watch it - I gradually began to assume that it was evil or something).
I turned 13 in 1999, and my first full episode was a syndicated rerun of 2F19 "The PTA Disbands". At first, I was a little off-put by the show - the character designs were alarmingly different, most of the humor went right over my head, I couldn't get used to the fact that there was almost no background music, and I thought the show had more continuity than it really did, so the ending really upset me (I remember thinking to myself "No school would loan out their extra space to a prison! Those poor kids are going to be traumatized!"). It took me a while to adjust to the show's mindset after that, but I was still thrilled to finally be watching the show that my classmates had sung the praises of for so many years.