Phyllo, you make some valid points, but to push back: at some margin, you are stereotyping every person you interview for a job. You have, at most, a few hours of interaction and a few pages of information about the person before you hire them. You stereotype them based on the past work, and you say “the average person who had job X would also be good for this job, so that weighs in this applicant’s favor”. That’s all stereotyping, it’s completely legitimate, and that isn’t at all at odds with the suggestion that race is illegitimate when used the same way.
It’s also not at all at odds with the statement that sex is a generally illegitimate factor to weigh, to say that sometimes sex is a legitimate factor. Moreover, it’s pretty easy to come up with cases where even something like employment history can be weighed in an illegitimate way: say someone applied to a job at a bank and on their resume indicated that they paid their way through college working as a garbage collector, and they were scored down because working as a garbage collector is seen as déclassé and the person is seen as tainted by it. That’s illegitimate, even though past employment is obviously fair game in general.
So, is age more like sex or more like past employment? I’d say it’s close enough to the latter. You dismiss the role of age-at-first-use of a technology, but that matters; people learn differently as they age, their thought processes solidify and consolidate and streamline, and they become very good at the things they’ve always done, and very bad at learning new things. While there’s some variation, it’s much more like claims about “the average person who had job X” than it is like claims about “the average person of race X”. And it applies to all technology, Facebook was just one I knew the timeline for. Google, Excel, Word, and Outlook are all essential in a modern workplace, and if you started using them in your late 40s you’re unlikely to get beyond rudimentary proficiency. Sure, it’s a probability, but then there are some people who have been cashiers for 20 years who would make great middle managers, but there’s no bias in betting on safe odds.
Look, there aren’t clean lines here. Age can absolutely be misused as a factor, and I don’t mean to suggest otherwise. But where with race and sex the odds that it will be used illegitimately make it safest to have a strong presumption that a particular use is illegitimate, that isn’t the case with age. Age tells you something meaningful about a person, and as often as not it’s rational and fair to factor it in.