[runako]

> the population of those places is on the verge of collapse.

That collapse has been underway for a couple of generations now.

>Has there just been a sharp demographic break within the last generation with all of the young people suddenly moving away, or are large numbers of 30-somethings moving from the city to rural towns?

Depending on how you count, young people have been leaving farms for 4 or 5 generations now. Part of this is down to the increased efficiency of agriculture, which means fewer people are needed to produce food, which means fewer agricultural jobs. Most job creation happens in cities now, so young people who are not inheriting a family business tend to move toward cities. Richard Florida and others have written extensively about this demographic shift.