I honestly think it's mostly a set of connected biases along the lines of:
a) "Progressivism", a bias towards thinking of "good tech" as the "novel/new tech". Or believing that success rides on quick adoption of new things, along with
b) A bias that believes that that the older you are the less likely that you have engaged with new novel tech.
c) A bias that believes that if you're still "just" an engineer and not management, there must be something wrong with you. Younger staff will accept "grey hairs" at the executive level but sometimes be bewildered by it at the individual contributor level.
d) A bias against more expensive talent generally.
e) A bias (among some) against talent that cannot be easily manipulated
f) A bias (among some) against people with children/families/mortgages/elderly parents, etc. that might "slow things down." (This one is maybe the trickiest because it can express itself in ways that are not at all intended to be malicious but end up with bad effects. E.g. organizing team offsites and gatherings which are hard for parents or committed spouses to engage with because of their obligations. And the older we get the more likely we are to have such obligations)
Most people who hold these biases hold them unconsciously. I personally look (or sometimes act) younger than I am, and get "surprise" from people when I tell them my age. Likely because they have bias about what that age should/could imply.
Age bias is mostly "correctable" in most decent human beings. Where it gets malicious is when it gets in the hands of management who have a cost focus.