Often it starts around age 40 but may be so gradual to be unnoticed for many years. Or, it can come and go with no psychological or physical explanation. Or for some it can be the results of physical problems, stress or emotional factors. But often, it is simply the natural result of aging. Complete impotence however, is usually the result of a physical problem that often can be treated.
Some medication can also cause impotence, especially hypertension drugs.
Diabetes is also a frequent culprit. Since most men don't talk about such things, a man may often think he is the only one and feel embarrassed with his partner. The good news is that for most men they can remain sexually active for the rest of their lives, but they have a adjust to softer erections and find it harder to get and maintain erections. But the response differs greatly between men. For most men without any specific physical problems gradually decreasing potency can be an opportunity to learn more intimacy.