Once a cock reaches maturity and finds its optimum size even loads of use wont wear it down or diminish its size - nature alas does that for us. As the age process kicks in so the flesh and muscle and meat round our lower body tends to thicken. Our cock remains the same size but now much is hidden inside the body, hence the idea that cocks get smaller as you age - its just the distribution of body fat that gives this impression, you cock shaft remains constant.