This thread is really amazing, in that everyone is giving advice as if it's gospel, when no one knows for sure how they'll react when facing age discrimination and termination as an older worker.
I have to confess that this whole notion is premised on assumptions I would question. Sure, there is some age discrimination, but with age should come the experience to pretty much define your position in the working force. I know a lot of old guys who make money hand over fist in tech, but then they never took themselves out of the game, tacitly or otherwise. Age discrimination mostly affects people who never had ambition and never built a CV (a reflection of a lack of ambition) over those decades. We've hired people in their 50's and 60's at high-tech startups, and it has worked out great; they have tons of very valuable experience that kids don't have, and the ones we hire still have that fire in their belly that allows them to take risks and blaze trails (their grandchildren notwithstanding). In practice, a lot of companies are hiring a combination of drive and experience, and not that many people have both.
The problem, as I see it, is that the business cycle continues to shrink and is becoming MORE entrepreneurial, not less. The problem is not age discrimination per se, but the fact that many people turn into clock-punchers once they get into their 40s, the working dead. There is less and less room in the economy for these kinds of people, and as an employee one can no longer afford to become one if you want to stay employable. Old employees may bring experience, but that the counterweight is that they have frequently lost their ambition and drive, which is also very valuable.
I've actually made my own opportunities more often than not, so I do not worry much about "termination" or even my future. And I'll probably be involved in some venture or another until the day I die -- it is in my blood. If we are going to sling advice here about staying employable, the one piece of advice I can give older workers is to find your drive again, even part of it. The experience is valued, but the frequent lack of drive is becoming increasingly costly to companies. Talking about age discrimination or insurance costs is really ignoring the underlying reason it is getting harder to get hired when one starts putting on the years, but most people find those reasons to be less threatening because it is not something they have control over.