"I have yet to hear a cogent argument on why folks in their late 40s or early 50s are not adequately represented in programming positions start ups or otherwise. What is unique about programming as a practice that make it unsuitable for professional beyond a certain age?"

Bear in mind that programming is still a relatively new field. There are simply more young programmers out there. We see ageism more because it's an option. You can start a startup consisting only of young people.

I predict that as the field matures, it will become less of an issue. Companies won't be able to ignore older programmers (or at least will be less able to ignore them) because there will be more of them.