This happened about twenty years ago and I still get hard when I think about it.
I had finished playing raquetball with a friend. My friend left without showering. I went to the locker room to shower and change. When I got to my locker there was an older guy, probably in his late 60's early 70's taking off his sneakers. We said hello and exhanged small talk. When he took off his underwear I noticed he had a huge cock and balls. It had to be about 10 inches and it was fat, cut with a small mushroom head.
I immediately got hard, I was naked too. He looked at me and said boy I remember when I could get a stiffy that fast, wish it happened that quickly now. I said sorry about the stiffy, it gets hard when it hits the air. He said I should be proud. I was a little embarrassed, because my dick is pretty small, about five inches hard. I said I wish I had a big one like his. He smiled and started stroking his cock. At first I just watched. He started getting hard. The head got purple and the cock stood out for what looked like a mile.
He said then do you want to play with it and reached out to jerk me. I then reached out for him and shot my load in less than a minute. I kept stroking him then he leaned in and started french kissing me. I got hard again. He then shot his load and jacked me off again. I saw him many more times over the years.