I have an entirely different theory, and it hinges on (1) self-selection of the type of elderly man who continues his gym-going and (2) dominance.
(1) Probably explains itself. As we age, we go to the gym less. The group that remains, and manages to continue gym-going into their 70s or whatever, is more overt.
(2) These overt men who remain are, by and large, alphas. Many are the same ones who, in high school, farted a lot, played a lot of grab-ass, and whatever else to knock classmates out of their comfort zones and assert dominance. As you grow up, you mellow, and those games become a lot more subtle. Dominance, for a 70yo man in a locker room full of fit younger men, is a far greater challenge, not to mention a much subtler enterprise. The simplest way to telegraph that you are still an alpha, not to mention to knock younger guys out of their comfort zone (which works or else you wouldn't have brought it up), is to whip it out and take it for a walk.