By that logic, surely every interview practice is justified? And the less related to the job, the better?
I could ask a programmer to build a wooden boat, become a proficient opera singer, or complete four marathons in a year.
Maybe! Quite often success at work is determined by getting crap done that doesn't require the skills you trained for initially.
I'm given to understand companies used to do this.
This idea has been tried. It’s called college.
Talking about age discrimination in particular, it's difficult since often older hires may not have the shiny new skill on their resumes or may have something indicating that this person may not be keeping up with tech (the bias). One of the weirdest things about joining a startup is realizing that 98% of engineers are in their 30s or 20s, while not having any engineers in their 50s, 60s or 70s.
In a sense, they are. As long as the interview practice is well understood by the applicants, it only leads to filtering the applicants that cannot pursue the requirements for that practice. Granted, the FAANG interview process and filtering only works because the SWE jobs are desirable enough.
I understand that the required practices and filtering also lead to a certain type of applicant. This is the main fault of the system (the lack of diversity potential).
You could, but you would get better results by posing more relevant challenges.
If you ask them to become singers, you'd get a whole lot of people with great voices, who may or may not be good engineers.
At least coding solutions to DSA problems has a fairly high (though not perfect) correlation with being a good programmer.