It is my understanding that they are to be found in larger numbers in older companies (Cisco comes to mind), and in defense companies. Having worked for a while in SV in a variety of companies, I have seen a small number of people in this age group.

There is also the fact that the profession itself is quite young, so it's possible that there isn't going to be a large enough cohort of programmers to notice, even if ageism were completely absent.

However, your assertion that there are loads is also based on just the exact same thing as my assertion is: anecdata. Where do you see large numbers of older programmers? Outside SV, perhaps?