Mine happened at 24. I had previously sucked off my best friend and swallowed his cum, but he never fucked me even though I had been fantasizing about getting fucked since I was like 9.
At 24, I went out to a Gay bar in a Chicago suburb with two friends. Then a guy about my age approached me and went right to the point: he wanted to fuck and get sucked. He was not particularly good looking, kind of thin and nerdy but it was his dom attitude that seduced me. I said yes and went to his place. The actual fucking was not very pleasant. Even though he had a small to average size dick, it hurt like hell. He took my ass raw and then he just sent me on my way. I could not have been in his apartment more than 30 minutes. A few days later on a Saturday, he called me and asked to come over to get fucked again and to buy some lube. I went as asked. The second time he fucked me I withstood the pain better but again he sent me home right after he shot his load in my ass. In all this I realized that at that point in my life I wanted a BF not a fuck buddy. When he called me again, I declined and he got pissed off. Never heard from him again. All in all, this experience reinforced my notion that I was a total bottom. To this day, I wondered how this guy knew I was a total bottom before he approached me even though I look very masculine.