I was also eight years old when the original "TEENAGE MUTANT NINJA TURTLES" hit theaters in 1990. By then I was naturally a big fan of the cartoon series and quite honestly it was hard knowing a boy in elementary school (and even some girls) at the time who weren't. The sure popularity of that franchise in the late 80's to early 90's was really something to have experienced, and can't be easily duplicated or explained. Even POKEMON in it's prime was nowhere near that insane.
At any rate, I don't honestly know how long the film had been in theaters when I saw it. What I do know was that on one weekend I was attending a birthday party for a friend from school, who was roughly 3 years older than me. It was a big shindig at his parents' house and the highlight was his parents and various relatives marching everyone about a half mile to the movie theater to see the flick; I think in total there were 20 of us. The theater was so packed that we all had to split up and sit pretty much wherever there were seats. When I'd usually gone to films with my mother I'd usually sit in the front with her, and the theater was so packed I was reduced to the back; it was the first time I'd seen a film from the back of a theater. Naturally the theater experience was insane. The audience would respond to seemingly every major line or beat in the film. It was naturally something unreal to see live action Turtles for a kid, and considering the film was PG compared to the cartoon it was like the "big kid" version of the Turtles. The tone of the film was dark, there were PG level "cuss" words like "hell" and "damn" in it. There were naturally a lot of differences from the cartoon - as it was closer to the original comics, which as a kid I knew little about - but it wasn't to such a vast degree that nobody could follow it. I distinctly remember one older girl sitting in front of me who was seemed genuinely concerned at the climax of the film when the Turtles were seemingly no match for the Shredder. This was not the often hammy, cheesy version from the cartoon who could be undone with a pizza to the face and a trip; this was basically the Darth Vader version. I also recall being surprised when Leo actually cut him with his sword; I sort of considered it "against the rules" to actually cut someone with a weapon in a cartoon, since naturally violence was mostly suppressed by censors on TV 'toons. At the end of the film, the reaction seemed deafening.
It was also a surprise to see Casey Jones play so big a role in the film, since he'd really only been in a few episodes of the TV show at that point. They were memorable to me and I've always thought Elias Koteas' performance in the role has often been forgotten by fandom a bit. Yeah, I imagine his bit in "TEENAGE MUTANT NINJA TURTLES 3" was basically for the paycheck along with everyone else involved in that farce, but that first time in was the balls. Plus, that mask prop was awesome.
I saw the sequel in theaters a year later, but the theater experience is probably something which has yet to be duplicated to me. The 2002 theater experience for "SPIDER-MAN" came very close, but still wasn't quite there. I'd actually seen "MASTERS OF THE UNIVERSE" and "SUPERMAN IV" in theaters when they were new, too, but this experience was naturally far superior. I went to quite a few films with my mother once I was old enough to sit still for them, but "TMNT" in the 90's is among my fondest experiences there as a kid; can't think of much which compares in my memory besides "THE GREAT MOUSE DETECTIVE".