Two definitive moments stick out in my mind.
1. In second grade, we were able to write stories about anything we wanted. I wrote a story very similar to the "Great Kimono" episode of TMNT. I remember being humiliated that I had to show it to my teacher, and the very confused look she had when she read it. I was terrified to show it to my parents, and I knew then that my interest was not what most would consider "normal."
2. My freshman year of high school, I spent the night at my friends house. He had some crazy thing called the "Internet", which allowed you to search for different things. I waited until everyone was asleep, and snuck downstairs to log on. I muffled the dial-up sound (remember those?) with a pillow, went to a search engine, and typed in tickling. Seconds later, I realized two things. I had a tickling fetish, and I wasn't alone.