> Places want to hire younger people because they're more apt to work longer hours for less pay.

This is the best light you can shine on the discrimination. Most often it really is managers taking their “seniority” literally. As in, they don’t want to take the risk their reports are smarter, more experienced or capable of replacing them, so they discriminate on the basis of age. It’s counterintuitive, but this feels truest from my historical observation.