It starts in the mind. One night where something did not work the way he expected. Then comes the loop - next time, he is already thinking about whether it will happen again. That thought alone is enough to make it happen again.
He is 26, or 29, or 31. He is healthy. Works out, does not smoke, sleeps well enough. There is no obvious reason. And that is what makes it worse - when the body fails at something with no clear cause, the mind fills the gap with explanations that are not true.