It may also be about the expected level of responsibility. A talented older worker will be in a position of considerable technical and / or managerial responsibility. If he / she tries to move out of their narrowly defined career, it may be difficult for a new employer to take the chance of giving them that much responsibility, but at the same time can't reasonably them as well for a position of less responsibility.
From what I've seen, moving within the same industry is not that difficult, but moving to a new industry is very tough for older workers.
An older worker who has lost their job (for any reason) can also be viewed as a risk. Sometimes "layoffs" really are about money but sometimes its a way of nicely firing someone. Do you trust that person to make critical decisions on your project?
Note, I'm one of those "older" workers and I don't like the idea that its very difficult to find different work, but I think I understand it .