I live in RTP and don't find the ageism thing to be too notable, but I do note that companies do like to hire people like themselves at times, and this is the one thing that happens in an interview that nobody talks about - what really is "cultural fit". Though, often, that's not hiring the guy that has been burned out by 40 years at IBM - not because he's old, but because that place can eat your soul out. Younger employees want to feel like they'll be able to make decisions too.
Computer Science seems prone to this because it's such a weird mix of creative and also deeply technical, it almost seems to make it a harder field to work in sometimes, because both instincts can't usually be expressed well in group contexts.
As for the perception of why ageism may exist, newer folks want to replace technology quicker, and don't have as much historical memory, so if anything, I think the challenge is to keep replacing things you are working on, and not specialize too much.
I think this will also get easier as the big wave of new C.S. folks get older themselves, but I built my last company with most of everyone in their 30s and early 40s.
Might vary based on what sort of companies.v Startups seem more prone to it to me, but my startup was pretty well spread out. More established companies are less likely to be that way.