hmmm... Iunno, there were many instances growing up that i played, "i'll show you mine if you show me yours" with a couple guys and girls, we were prob 8 or 9 years old, the age you don't really know what sex is.
I remember one guy, we were pretty close friends when we were younger, and I had a big elaborate tree house in my backyard that my dad had made, and me and him would go up there, and fool around all the time. Humping each other, sucking, jerking, but not really knowing the terms of what we were doing, we just said humpin for all of it. I got in contact with him again a couple of years ago, and one night on msn we were talking and i casually brought it up, and he said that he remembers doing it, but he don't swing that way anymore... who knows though, he might have been afraid to admit it.
I consider my first real experience, that I knew what I was actually doing was when I was 12, and my neighbour was 12 also. We were best friends, and one day when he was over after school we were playing the N64, and after the game was done, he stood up and told me to stand up. So I did, and he got behind me and started grinding my ass. We both got hard and we played around with each other dicks through our pants... Each day it progressed, until we pretty much did everything. We had MANY sleepovers after that, having lots of fun at night.