I think my tickling kink started when I was about six or seven, even though I didn't realize it at the time. My friends and family would absolutely torture me. I'm... well, I'm pretty ticklish so of course I was going to get it bad. I'd always do cunty little things to annoy people into pouncing on me. If that didn't work I'd simply ask to be tickled. Nobody ever said "no" to that request. I was also a notorious "tickle monster," as well. Moreso than I am now at least. I was always deeply fascinated and excited when I'd see tickling in cartoons, movies, comics, etc.
My earliest memory of realizing I had an above-average interest in the subject came to me when I started reading R.L. Stine books in elementary school. I'd read tickle scenes over, and over, and over. And his books had quite a few such scenes. One book was about a couple of friends who get abducted and eventually tickled to death by aliens, if I remember correctly. In another, a guy had to defeat a monster by tickling her/it's feet. There was a catch though; apparently he couldn't actually defeat it. He had to keep tickling it or else it would kill him. I always felt sheepish, like I was "doing something wrong" because I enjoyed it. The Where's Waldo books are also pretty saturated with tickling. I loved reading those. Hell, I still do!
It was a terribly cold,wet, stormy fortnight in 1998 when ol' pops decided to connect to the internet when things really, really changed. I was only nine or ten at the time. I would search for anything and everything I possibly could (except for porn, honestly!), when one night I searched Lycos or AskJeeves for "tickling." Pandora's box had been opened. I found Wryel's Wrealm and Swtickle. Swtickle was a gold mine, I couldn't believe it at the time. Wryel's Wrealm was my favorite, but those were the dark, unrelenting days of dial-up, and it took for-fucking-ever to view anything there. But eh, yeah. That's about the time I finally made the connection.