Probably since birth. But realistically like many others have said, around 4 or 5 yrs old. When i was that age I had a certain feeling "down there" when I would see women's feet (especially with nail polish) and when seeing them tickled it was a more intense feeling "down there", which was sexual arousal but didn't know it that young. Would have fantasies about tickling the feet of teachers, mothers of friends, female friends of my parents, etc, etc.
I could actually write a whole book on my experience and struggle with this growing up, but to keep things basic....all my growing up in the late 70's through the 80's, then into my early adulthood in the early to mid 90's I thought I was a one off freak of some sort, even though I came out of the tickle closet so to speak in 1996 thanks to a real awesome woman I was messing around with at that time).
It wasn't until the very late 90's/early 2000's though when the internet came about that I found that this fetish was an actual fetish that millions of others around the world had too (enter the discovery of the TMF and a few other sites), and that there were even videos to wack off to lol Only wish I knew all of this decades earlier, woulda made life a lot easier.