I'd have to say 6 years old for when it started in me. I was playing with a mate at his house and his older sister wanted wanted to join in so we played games like red rover crossover. Then we played this game where I was on the floor and they had to get across without me tagging them. I got his sister and I can't remember who instigated it but we both started tickling her. We did this on a couple of other separate occasions. I always went for and liked to tickle her feet. She never rejected the idea of the game either which is interesting.
After that first time though, something changed, something had awakened in me. I was so excited, I had butterflies for ages, even after we stopped and my mum came To pick me up. Even though I was only 6 I knew that it would have been weird to talk about, so I was very private about the fact I liked tickling. Ever since then, I have chased that feeling 🙂