As with several other previous responses, I am going to maintain a double standard with manhood and masculinity - specifically, that you as an adult male have the power and resolve to NOT be "victimized" by the incident. In other words, let's say that this old man's behavior is pervy at worst.... He probably would not be able to overpower you, and if you laughed at him or confronted him, he would probably have zipped up his pants and ran out of the bathroom i embarrassment. As an adult male, you have the strength and hopefully the character to not be diminished by his behavior, even if it is unusual.
I am in my 30s, and even as recently as 6 months ago, I have had "pervy" approaches in the men's room at public places (and work). I was in a stall and was surprised by a gentleman who reached his hand under the stall and started to stoke my ankle. Caught off guard, my silent reaction was almost to scream out, but I caught myself and realized how ridiculous the scream would have sounded in the men's room. I let the gentleman stroke my ankle for about 10 seconds, and then I simply said, "No thank you". The man ran out of the bathroom -- the matter was closed. I wasn't harmed, I could defend myself if the situation got worse, and I realized how sad it was for a person to resort to the men's room for a sexual thrill.
There are far greater threats in the world than a seemingly lewd, man-to-man approaches in a bathroom. You may or may not have stimulated his jack-off response, but you certainly weren't harmed...