My lifelong obsession is anime. It started when I was I was 8 and got swept up in the Pokemon craze of the 90s, and has been ongoing ever since. The only thing that changes is the series I'm into. I tend to obsess over two series at a time for the span of 1 to 3 years. For example, age 11 - 13 was Rurouni Kenshin and Inuyasha, age 14 - 15 was Naruto and One Piece, and age 16 - 18 was Hellsing and Trinity Blood.
However, it did go on hiatus for about one and a half years (age 19 - 20) because of developing a new special interest in the live-action movies of Tim Burton. I thought that I had "outgrown" anime, but I was wrong, as it has recently returned to the forefront of my mind. I'm still a big fan of Tim Burton's movies, but for right now they are secondary to my interest in anime.
I'm still trying to find a couple of anime series to obsess over. There are a lot of them that I like but I'm still trying to find two that will dominate my thoughts.