One way that older people can help keep themselves marketable is to have management skills. I can tell you from experience that companies are always starved for managers. Especially growing companies (and those companies are usually willing to pay a premium for good managers).
For some reason, precious few workers decide to take a management track in their careers. As a result, many of them end up dead-ending themselves like this guy evidently did. Good managers are much less expendable than worker bees. And once you are in management, age actually helps you. Earlier in my career, I was passed over for promotion on the basis of my youth and inexperience! Especially when you get into middle-management and the VP level - companies prefer to put forth seasoned, older people.
However, many workers are just not interested in management. I guess they are either intimidated by the job (feel they can't handle it) or they feel that they are being disloyal to their fellow workers by taking such a job. But being a manager is not as tough as it looks. It does require a thick skin, the ability to handle all types of people, strong organization skills and a positive can-do attitude. If you have those traits, you have the core skills to be a very effective manager.
Once you have established yourself as a manager, you can manage in just about any working environment, giving you many more career choices. For example, if you are an IT worker who specializes in Novell, you are pretty much limited to that niche until you train and get experience on some other platform. However, if you manage an IT department, you can manage an IT department anywhere regardless of what platforms they use. Hell, if you are a superb manager, you can manage a supermarket, a factory or a dry-cleaning business. Because no matter where you work as a manager, managing is pretty much the same. You manage people, profit, and control expense. So whether your company sells soda, computers, cars or services, all you need is a primer on your companies core functions. All other facets of management are transferable.