There are 4 major movie franchises comprising basically the top of my catalog of taste. The absolute pinnacle is Star Wars. But closely following, are these three: Alien, Predator, Terminator. All three introduced to me by two of my cousins around the age of 7 or 8. ... So that would have been around '91-'92. I'm big on sci-fi but I'm also big on the macabre. I was always a monster fan since as far back as I can remember... started with the classic monsters... and dinosaurs... dragons. If it was to be feared, I did not fear it, but embraced it. Might have something to do with too much emphasis placed on Halloween as a holiday when I was young... Idk.
Point is when I first saw Aliens I was an instant fan. Alien itself didn't work for me because a kid that age doesn't really have much attention span for a film with so little action. I then discovered Predator through the older brother of my 2 cousins, who owned a copy of the full graphic novel of the original Batman Vs. Predator, and loved the creature's design instantly. After later renting the original and seeing the epic Arnold Vs. Pred fight for the first time around that age... well that sealed the deal. Instant fan again. To me at the time it was more like Terminator Vs Predator, because I had also just been introduced to that concept as well.
{EDIT} I need to add something... I understand when very young fans denote AVP as their beginning. But I seriously hope there are as few adults as possible that would follow suit. As much as I love the concept of AVP, the AVP movies themselves were both terribly disappointing to me.