This is often thought of as pure age discrimination, but it's more complicated than that. Blue collar workers at 60 may not be able to physically keep up with their old jobs. They don't slot well into union shops that are designed to hoover up young workers and keep them for life. And its harder for them to work for minimum wage, because they have obligations.
This is even more true of white collar workers. Especially as they move up the management tree, older workers get a lot more expensive. This is in part because we've cut a sort of society-wide bargain in which people in most jobs are paid more as they age to offset rising expenses. But also, older workers have more skills.
In general, more skills is a good thing. But in an increasingly specialized society, those skills are increasingly specific,. The more skills you have, the fewer jobs there are that match them.
The New York Times chronicles the unbelievably extensive process of finding a baby-boomer executive a new job in the current recession. That's a story there will be a lot of in the next few months. Expensive older executives are often first fired, last hired.