I don't know, I seem to have the opposite problem. I get contacted by recruiters from big silicon valley companies on a daily basis (Apple, Google Netflix, Okta, and just endless other companies in SF and Silicon Valley) and they just won't leave me the f*** alone.
I'm almost 36 years old, so above the average. Also where I work there's plenty of old people around, some in their 50s I'd say.
I think the problem isn't necessarily age discrimination, but the skills silicon valley companies and web companies need are simply different to the skills us older folks have. We cut our teeth on older tech (Physical Hardware, Main frames, SANs, building our own Datacenters and Networks, Compiling Kernels, etc) that's been replaced with new modern cloud stuff (do it all on AWS, used hosted services, etc.)