Thanks for posting this, Jeanne. As Gerald's beard is even whiter than mine, I'm inclined to trust his analysis!
There are also other factors that may result in covert discrimination against older programmers. If you've worked in a company a long time, you may have become the only person who still understands how a particular system works, so you may find yourself pigeon-holed and denied opportunities to expand your skills/experience. Also, many companies provide few senior technical roles for experienced techies who want to progress their careers while staying involved in development without being pushed into management. That may be one reason why my own experience is that freelance contractors tend to be older on average than their permanently employed counterparts: as age seems to be less of an issue when a company is only hiring you for 6 months, and when you've reached the top of the tech pile as a permie, the only way to improve your financial and professional prospects may be to go contracting. Plus, there comes a time when many of us simply can't be bothered dealing with office politics any more and just want to get paid for doing a good job!
Meanwhile, back in regular employment, as Gerald's previous post suggests, the fact that your experience means you take on harder jobs may actually count against you: if the youngsters seem to finish their less challenging tasks faster than you, managers may decide you're not as "productive". Many managers also seem to equate programming with typing - if somebody is typing more/faster than you, that is taken to indicate that they are doing more work. In fact, that may be true in one sense: by taking less time to think before hitting the keyboard, they may indeed be having to put in more work to achieve a particular goal.
However, there are some confounding factors at work as well, at least here in the UK, where there has been a steady trend of companies firing their older, more experienced (= expensive) UK-based staff and replacing them with offshore teams or cheaper "onshored" staff from low-wage countries. If you're 40 years old with a family, mortgage and pension plan to support, you'll never be able to compete on cost alone with a young graduate, especially if they're offshore, and too many companies are only interested in the bottom line where the quality of their staff's skills and experience doesn't seem to matter. This short-term thinking has effectively hollowed out the technical skills base in many companies, as experienced staff have been fired, yet this not created many new opportunities for the next generation of UK-based youngsters to develop their own skills and experience in the IT sector either: youngsters can't find jobs, and even if they do, there are no longer enough experienced technical staff for them to learn from. Many workplaces seem to end up with a skeleton crew in the UK while much of the work is conducted offshore. So ageism is not the only problem faced - by young and old workers alike - in the UK IT industry.
On the plus side, here in cyberspace no-one can see your grey hairs, and there are plenty of smart folk of all ages here at JavaRanch for us "mature" programmers to keep learning from!