For me it happened extremely gradually. My fetishes started by smelling babysitters feet when I was about five. Tickling came much later.
I've posted about this before, I think. The first time I ever recall paying attention to girls being ticklish, was the summer when I was 12, at sleep away camp. That summer, my bunkmates and I tickled the feet of these two girl couneslors, Christine and Adina, who were both 19. Both were very to extremely ticklish. After that, I began reading articles in leg magazines, and Penthouse Forum Letters, about tickling, in my teens.
I didn't realize I was into tickling "In that way" until 1997. I answered an ad in a leg magazine, of a straight male into tickling. He responded. We both had female snail mail pen pals, and used to send them letters asking them what we called "Our Favorite Question", asking if they were ticklish, and their experiences. I got several dozen responses between 1997, and my friend's death in 2002. My correspondences with him, and my excitement at the answers I got to my ticklish questions to the girls, validated the fact that I had a tickling fetish. In fact, I still retain every letter from every girl that I ever got an answer to tickling questions from.
My tickling fetish is probably different from others on the site, in that I didn't realize it was a fetish until much later in life. My female foot fetish started much earlier. Yet, I love how it came about, and, if I had never known my pen pal Don, and written those letters to female pen pals, asking them tickling questions, I never would have realized my interest in tickling was the fetish that it is.
Mitch