"Jelly juggling" in my back garden with my school "girlfriend" (we were young, it wasn't serious stuff) for a charity event when we were... 14? It was just a silly fun thing to do.
Then when I was 20 with my 18 year old girlfriend, a few things like covering each other in shaving foam, and a few photo sessions I did with her in my flat.
I was in my 20s when I first got messy. I had a lilac ballet leotard I used to train in and I decided to fill it with a tin of beans and two eggs I "pinched" from the kitchen as I still lived at home with a parent. I remember it so well sat in the empty bath and then trying to get the beans to go down the plug hole
Pie booth in college. I had always been interested in mess secretly but I never had a chance to experience it myself. I was asked to volunteer for homecomings county fair. They mentioned what they need volunteers for. When they said pie booth, I thought to myself, "It's not the kind you're hoping for. It'll be a pie eating contest or maybe just serving slices of pie." But I signed up anyway. Then I found out it was exactly what I had always dreamed of. Being a pie target to my classmates, strangers, and friends. I was probably pied a dozen times that day. Not bad for my first experience.