No real age for it to happen.
Depends on your health. I know blood pressure (and blood pressure medication) can make things difficult for a guy 40+. I hear that diabetes (perhaps their medication) can also cause problems. Steroid use also causes problem in men over 40.
But basically, it comes down to how healthy you are, and your lifestyle. It also could be based on how happy you are in your marriage (as in, it can get harder to get harder if you aren't happy in your marriage).
So, keep your marriage happy and healthy (and you stay fit too), and don't worry too much about certain, um, failings.
Oh, stress is a killer too. Try not to get too stressed out, it definitely causes problems with both marriage and health and sexual performance.
But, of course, you understand that the above has all happened to a good friend of mine, right? And that I'm just speaking from his experience...
