I fit this description as well. Who knows what exact age it was when I started to feel 'that way' about tickling? ... I know it was before I started going to ( nursery ) school , though.
I can remember being that young & whenever a cartoon or whatever that had tickling in it came on television ... particularly foot-tickling ( my foot-fetish has been around since forever , too ) ... I would get that special feeling & ... uh ... lil' TKLMAN would stand at attention. I of course didn't know what the heck that meant at the time , but for some reason , I knew enough not to tell anybody about it.
Obviously & eventually , my love & fascination for tickling has just grown stronger over the years. I guess I first realized that what I had was a "fetish" occurred sometime in my teens.