I suppose programming as a particular profession may have this dark secret; I'm not a straight-up developer (meaning I'm an IT generalist who does develop but usually has some sort of hybrid job including mgmt).
However: I know lots of 50-something developers who have no problem maintaining employment and contracts. They are top-notch; their skills are current and their proven track record and accomplishments are what get them gigs. Companies want them because they're less of a risk -- instead of guessing whether the young guy will develop the right skills, they can take much less of a risk and get someone who's already proven himself.
Observing older programmers within my circle, they tend to master a few specialized skills that's hard to get from the hardcore and enthusiastic 20+.
There are a lot of subdomains within programming. Mastering things like Unix tools, patching certain web servers, knowing the kinks of specific RDBMS and/or filesystem, how to design batch processing, makes you a lot harder to be replaced by those 20+.
Not all 20+ guys are hacking on PyPy or Firefox. Most of them are Rails/PHP warriors. Competing against them is a lot easier if you master skills that can only be obtained by experience.
How many of them are normal, full time permanent employees vs. contractors? The major issue I perceive here is that things are ugly for people wanting the former and many aren't cut out for doing the latter.
Choice or necessity? As far as I can tell, if you want to keep programming with anything resembling good odds you can make a major name for yourself, become a multi-client consultant, specialize in embedded programming or get a serious government clearance. I suspect your example is survivorship bias with the 2nd option and perhaps some of the 1st. The ones who didn't chose contracting (and didn't get into either of the last two options) probably just aren't programmers anymore. The official statistics (age vs. profession) suggest as much.
However: I know lots of 50-something developers who have no problem maintaining employment and contracts. They are top-notch; their skills are current and their proven track record and accomplishments are what get them gigs. Companies want them because they're less of a risk -- instead of guessing whether the young guy will develop the right skills, they can take much less of a risk and get someone who's already proven himself.