I'm just a demi-lurker, but I'll give it a go...
D&D - 1981 (Purple box basic set)
Star Wars - 1977
Earth sea - Still have never read to this day.
H.Potter - 1999
Magic The Gathering - Whenever it first came out, but naturally I didn't try it until
after all the black-border cards had been sold out. Got out of it after the "Legends" expansion.
Marvel/ D.C - Superman and Spider-Man are so ubiquitous I can't remember a time when I didn't have something with their logo on it. I didn't start collecting comics till the mid-80's though when I seriously got into X-Men and collecting back-issues of Moon Knight* and Micronauts**
Bonus Round:
Star Trek: I'd seen re-runs with Kirk earlier, but it wasn't until TNG aired that I really got into it.
Galactica: I watched the original; I still love the intro theme sequence. I was never that big into it, though. I also watched the 2000's series... which we'll just say I liked the first half better than the second.
Dr. Who: I remember watching re-runs with Tom Baker in the 80's, but I didn't give it a serious go until the Eccleston series in the 2000's.
* I felt like the only kid who'd ever heard of the character. Imagine my recent delight when they made the D+ show! I had waited a lifetime!
** I had some of the toys, so traded an issue of Fantastic Four for
three Micronaut comics. The original run is a pretty cool sci-fi saga, but the licensing rights are so mucked up we'll never see a proper adaptation. Guardians of the Galaxy + a dash of the upcoming Quantumania will probably be the closest I get
