It happened when I was 12. My best friend and I got some of my brother's porn and were looking at it. I went to the bathroom and when I came back to my room my friend was grabbing his pants.
I told him I jerked off. He said he didn't know how, so I asked if he wanted me to show him.
He said yes and pulled his pants down. I jerked him off and told him to jerk me off so we would be even. He did, and I loved it.