I think this stereotype/assumption (east coast, FWIW) - is probably due to the radical uptake on the field. There are a lot of young people, and that's starting to die off a bit more as computers are "a given" and those people themselves get older.
Simply put - there will be a LOT of people in this bracket in not too long, and a lot of useful work to do. This group will also have some of the most architectural experience.
My last company had most of the developers in their mid thirties (as am I), for instance. Admittedly, that's not over 50, but larger companies tend to skew a bit higher in that direction.
New startups also generally pay less, which is also a reason more experienced engineers sometimes don't go to new startups - or even seek out larger companies. While there are some aspects that may be boring, larger companies also have more resources, sometimes have more interesting labs, and while you can see and change less of the system, you are less apt to have to deal with certain parts of the system you don't want to deal with too - because there's more specialization and organizational seperation.
As people get older, there's simply not going to be a management job for each of them, and not everyone is going to want to do management - and that's great. Management is not "better". I suspect the ageism will go away simply as more people get older and realize they too are not just out of college anymore.
And having that experience is good for everyone.
Hopefully it also slows down the rate at which javascript programming frameworks are replaced as these folks also get tired of replacing things every two weeks :) Ok, kidding on that last part.
Anyway, my guess is ageism in tech isn't really proven yet. it's a theory, based on the wrong assumptions.
However, yes, if your company is still shooting nerf guns at each other, and irresponsibly managing release schedules to be in constant crunch time, a wide amount of people aren't going to want to work for you.