They were franchises I was aware of, as a kid in the 90s, but never allowed to watch. Given all the early movies had 18 certificates in the UK, my parents ensured that as kids, my brother and myself didn't see them. As an older teen, my dad finally introduced me to the Alien quadrilogy box set, and I think the Predator movies (only 1 and 2 back then, with AvP having been out for a short while) after.
It was the Alien franchise I invested in far more. As AvP was rated 15, and I was 15/16, I saw that, and loved it (despite the general consensus, I still find it a decent movie; I watch the extended edition, and it's a lot better than the theatrical version). So it really came more from my dad, introducing it to me at an age where not only could I handle the horror, but also understand the themes, particularly in the Alien series, where the real villain is unchecked capitalism, rather than the alien creatures.
I was hooked ever since. I enjoy them all, and have my gripes about them as we all do, but that first Monday night when I saw Alien for the first time just had me in its grasp, and has never let go.