Is there really widespread ageism, or can that be explained by there being 6x more programmers now that 20yrs ago, by the high pay allowing early retirement for those who want it, and by people with more experience self selecting away from "opportunities" which might succeed in attracting new grads who don't know their worth?

Not to say that specific instances of ageism don't exist or aren't a problem, but in the context of "planning for it in my career" I haven't seen evidence that would make malice more likely than other factors for tech's age distribution. Does that evidence exist?