Solo is a great site!
A few years ago, my family went to this campground as our little vacation for summer. The campground was great with its location near the ocean and its bike trails, but the highlight of the campground was the showers.
In the campground, the men and the women had different locations for showering. In the men's shower area, there were individual stalls lined up in a row inside the enclosed shower area. Anyways, I went to shower one day after coming back from a swim at the ocean. While I was showering, I dropped my little shampoo bottle when I was shampooing my hair. As I went to go pick it up, out of the corner out of my eye (and it was very difficult to see), I could slightly make out a guy in his late 30s pleasuring himself in the shower stall next to me.
Immediately, I froze and was suddenly very turned on and very interested. (Naturally, as a 13 yo, an older, the mature penis of an adult is very erotic.) As I discreetly peeked under the shower partition separating our shower stalls, the guy was concentrating on his balls, rolling them around in his hands and occasionally stroking the shaft again. The guy was circumcised and about a good 6' long.
Although I was certainly enjoying myself with the show, the guy could sense that I was peeking at him. Unlike some of the other stories here on solo that entail the guy 'returning the favor', the guy that I watched wasn't happy at all that I was peeking at him. In fact, after I left the shower after watching him, on the way out, he shot me a very dirty look.
Although he was probably pissed off at me, I enjoyed watching him masturbate to an extensive degree. A couple of days later at the same campground, I caught a younger guy in the shower next to me jerking off as well. I didn't realize that older guys are as horny as I am!