I think it's probably a combination of physical age and degree of acclimation, which tend to be correlated. When I first went to nude beaches in my 20's, I was so turned on by being seen naked and seeing so much nudity around me that I had to lie on my stomach for at least 30 minutes before I could even think about rolling over. Even then, it usually took several false starts. Gradually, I acclimated to the point where I can usually just strip naked in front of everyone and keep it below a semi.I wonder if age has anything to do with NOT getting a boner on the beach. I use to get semi's but nowadays I get I'm so use to it (being naked) that I just don't get hard from it. I'd have to play with it and I don't because it's just not cool IMHO.
I still have my moments, though, well into my 40s. I just went to a nude beach today for the first time this summer and had several moments where I had to roll over or run into the water. Even though I'm naked in front of men several times a week at my gym, which has open showers, the nude beach is a whole different scale..i.e. when I start to think that dozens of men can see my dick at any time, rather than a 10 or so in the gym shower, I'd start to get aroused. Again, this goes away with acclimation.