I am 32 now but this occurred when I was 16.
I had always slept with my older brother who was six years older than me, and often felt the bed shake and he would be moaning a lot. Of course I was very curious and soon realised that he was wanking.
One night, I cautiously kept moving my hand over his thigh until I could feel the bulge in his pyjamas. Excitement made me put my hand through the fly fold and feel his hard cock. Suddenly he asked why I had done that, and I replied I wanted to feel another guy's cock to see what it felt like.
He didn't say anything but pulled my hand back onto his cock and by instinct I began to move my hand up and down his shaft, it was a wonderful sensation and even better when he came and covered by hand in cum. Afterwards he turned towards me and took my raging hardon into his hand and repaid me with the most tremendous jerk off I had so far experienced. I shot cum so far it hit the bed headboard, my face and neck. There were lots more times we had mutual masturbation sessions right up to him getting married, but none since.