The fact that this lowering of the libido is so widespread implies something about society. There must be a lot of older blokes out there whose health is declining for various reasons. Of course men are notorious for not looking after themselves. And they do seem to get worse in this regard as the years wear on. But it can't just be that.
Perhaps it is related to men's career progression. They do tend to do less physical work the older they get. That is understandable, since some jobs are done far better by fit young blokes. But this means that they often end up in office jobs working quite long hours. These hours get longer, and more stressful if they are climbing up the career ladder, too. Subsequently they end up stressed out, sedentary and overweight, with no time to exercise. No wonder their health deteriorates!