I wouldn't know where I started - most certainly I was fairly little.
I kind of noticed some uniexpected reactions from some scenes, like Linda Lovelace getting tied in the Wonder Woman tv series or girl characters getting beaten in fighting games, but the effect wasn't so strong that I actually noticed I felt something resembling arousal.
I realized I had a serious fetish going in early high school - saw Warner Herzog's Nosferatu and enjoeyed it as a quality movie, until I saw the final scene where Isabelle Adjani lures the vampire to her side and tricks it into biting her all night. It had more effect on me than anything I ever saw - I think it's the reaction "normal" kids get the first time they see actual porn.
My main fetish still is about vampire scenes - girls getting immobilized and slowly drained of blood/life force/whatever, or dragged-out ryona scenes where a girl is slowly beaten, squeezed, strangled... method doesn't matter, only that a girl gets slowly weakened by someone, that's the thing that clicks with me. Actual violence such as mutilation etc, I despise, and there's not ambition to actually hurt anyone. Sex doesn't have much to do with it either, unless it fits in the humiliation theme.
Never had problems with fetishes, but I had a lot of trouble finding material that interested me, and ended up thinking my interests were pretty unique. Saw some people discussing guro and made my searches - let's just say my visits to the (actually very nice) community of Gurochan weren't exactly disgust free. Only recently I actually farmed youtube for RE clips and saw several related videos were tagged "ryona", tried looking for the word and discovered I wasn't alone.
No one in real life knows of my condition or has the slightest idea. Impossible to explain. First time I talk of it, thanks for giving me a place to do so.