Most juniors fails white-boarding as well, so we can assume that most seniors would fail whiteboard interviews when they were young as well. So I think this isn

Most juniors fails white-boarding as well, so we can assume that most seniors would fail whiteboard interviews when they were young as well. So I think this isn't ageism, just people with lots of experience who don't want to admit that they weren't the top X% of their cohort. Of course it is easy to believe you could have passed them back then due to the Dunning Kruger and the "Good Old days" effect, but that doesn't mean that it is true. If your argument is that you barely passed those interviews a decade ago and it takes too much energy to practice again, then I'll say it is a feature. The more we discourage less capable people from joining the better, don't you agree? Of course this all assumes that white-boarding is a good indicator, but the companies administrating them has pretty good statistics that they work. Also it doesn't have to be on a white board, at least Google lets you write on a Chromebook since a few years back. The important part is that the candidate has to solve and code a non-trivial problem not available in public (meaning they can't have seen it before).