I've worked in C, embedded and server-side, for about 15 years of my 17 year career. Companies I've worked with recently are having trouble finding C-capable developers, particularly C developers who have the first clue about 'modern' developments like CI and CD, or git.

C is a language invented in the 70s. If you kept up with thread APIS when they became usable in the late 90s you could quite easily have had a 40 year career in one tech stack, and be looking at a bit of consultancy by now (if you haven't moved in to management or other areas).

So even without keeping current on frameworks and languages, the idea that knowledge becomes obsolete in our field after just a few years is wrong. You do need to keep an eye on wider trends and newer tools, but still.

That said, there is still ageism in this arena. And I'm breaking out because, really, there are better things to be doing in 2017 :)