To work is where I go (59). You won't meet a lot of people like me because when I started (1981) there were way fewer programmers than today plus some became obsolete, some became managers, and some got burned out. So the chances of meeting someone like me if you started in the past decade is not high. This of course makes it hard to decide how much age discrimination there is as it's hard to quantify, unlike gender…
So if we look at 40 it really is a dividing line where of the few people that where doing development a small fragment of that group actually started developing in what would be somewhat relevant technologies today. Of that group, possibly only in the thousands, attrition has taken it's toll (the .com bust took a lot of them, which gets missed a lot in these discussions).
Now we have all had that bad interview, with a bad company that puts some kid fresh out of school in a lead role and he grills all new applicants on theory that is in no way relevant to what we actually do and anyone in the industry for a meaningful amount of time, has long purged that info from immediate recollection. But I don't see that as an age discrimination problem, I see it as a bad management problem, and a willful lack of understanding about technology and technologist on the part of business people (for those reading, first technical hire can make or break your company). Fortunately it has been my experience that most of those companies don't stick around for too long.