I was 15.
I was on vacation, and at the gym (I was a gym rat then, just as I am today) and in the showers I see this guy with a massive cock. And he started playing with it and showing it off. Needless to say, I wanted it. It probably hung 6/7 inches. But when we went in the sauna together and I slyly slid my hand over, I made it 10 inches. The only thing we said to each other was "How big's your cock?"
Ever since, I've been hooked.