I wish I could remember where mine started, might've been around 11-12 area.
My memory is bad for a multitude of reasons, I've always at least partially blamed it on the time an online friend sent me a huge photo of the Black Dahlia when I was at a fairly young age. It gave me nightmares, scared the hell out of me, so I forced myself to stare at it. Every day, a couple minutes at a time, forced myself to be okay with it.
From there, I don't know where it sparked in particular, but mine varies from a lot of you guys' in that its not so much the dead body for me, its the violence. I've been in innumerable fights since I was in about second grade, and by the time I nearly killed another classmate, it definitely was more than just self-defense.