Awww memories hehe
I actually started pretty early in life...
When I was 4, my mom would babysit a 4yr old boy (I have always played with the boys).
Anyway, we'd play in the bathroom, have water drinking contests etc. once he went to go pee and was standing up of course and myself being so informative, told him he was supposed to sit down silly!
So he'd sit and I'd hold his willy down.
Ok now that's VERY embarassing to tell anyone LOL
I've always known the difference between girls and boys...just sorta grew up that way.

Anyhoo, first "sexual" would be....well dry humping last summer with a friend i didn't really like, it never got too hot and heavy though.
Serious stuff would be just a month ago with my bf.