I've been a part of hundreds of interviews for dozens of engineering positions at a variety of companies and organizations. While I have witnessed clear-cut ageism with qualified older candidates, it's with the distinct minority of older candidates, and typically only with borderline ones. Compare to even the most qualified female candidates, where there's almost always a strong undercurrent of sexism, or non-Asian minority candidates, where there's almost always a strong undercurrent of racism^.
So I won't deny that ageism is a problem but - in the case of white dudes - it's the jolt of a sudden uphill at the end of a couple decades of easy slightly downhill coast. Compared to continuous vertical climb that women and non-Asian minorities have to put up with in engineering, it's a pretty modest problem that the affected have both ample time and opportunity to prepare for.
^- The tech industry isn't, at least in my experiences, blatantly racist/sexist these days. Anecdotally, women and minorities have an easier time making it through HR/recruiter screening. It's a more subtle and pernicious effect in tech interviews, where correct answers are overscrutinized for flaws and even marginally incorrect answers are blown out of proportion. Never seen this with older (white/Asian, male) candidates.