For me, it was 20 years ago this March when I was eight years old, going on nine (I'm gonna be 29 this May)
I first read a DK (Dorling Kindersley) book (rented from my local library) about Titanic in early 2001 (it was part of DK books Eyewitness series, which I'm sure many of us read when we were young), and not long after I first saw the 1997 film by James Cameron. I was very impressed with it, particularly the scenes where we see the ship alive and sailing again.
After that, sometime later that year, I saw 1958's A Night to Remember and some of the famous 1994 documentary series The Complete Story, and I became even more entranced with the story of Titanic.
And since then, my interest in the subject is still as strong as it ever was, with me constantly learning new things about the ship. So much so that I'm writing a novel set on Titanic's sister Britannic, and I've been working on that since November 2008.