> How do you individually address an issue where the hiring manager is making a generic decision based on age unrelated to your individual performance (especially in hiring where they have no idea how many hours you personally work)?

There isn't evidence of this, but even if you believed that were the case, you combat it the same way you would any other weak point of your value proposition. You address it head on in a tactful way that resolves those concerns without having to be asked explicitly.

That could include interest and experience in an emerging tool or architecture (combats stereotype that skillset is outdated), casual commentary on the latest Game of Thrones or Silicon Valley season (combats stereotype of bad cultural fit), or a true passion in solving the problems the company sets out to solve (combats stereotype of clock-in clock-out worker). It could also include adjusting your salary expectations more in-line with what your competition is expecting (if necessary).

The possibility that ageism is not the only cause cannot be simply ignored.

True. What we need is a"not hotdog" style app for tech interviews. Simply photograph the face of the candidate and wait for the app to tell you if they are too old for the job. It needs a hip name. Suggestions? (this was a bit of light humor about a serious and important issue)