Very true about the lack of a number, but that's part of the problem domain. A number that is healthy for one person is associated with symptoms of hypgonadism (that respond to treatment) in another. Complex dynamic systems are involved, and serum testosterone level is a single, crude measurement.

That's why I'd recommend anybody to get their levels of testosterone and other hormones checked periodically, so that any sudden change can be identified (and perhaps we can collect data to better understand the epidemiology of such sudden changes).