If you've ever sat in a hiring meeting, you'd know it's straightforward discrimination, but it's positive discrimination in favor of young people . It goes like this:

"Old guy here is a low/mid level person after 20 years, so he clearly isn't a superstar. Young guy here is low/mid level person, so he has potential to be a superstar."

Hiring committees, smothered by the legal ass covering around race and sex discrimination, never even think twice about making this discriminatory argument. It's even officially enshrined in hiring plans as "expectation of trajectory", a rule that only applies to old people. And before you say "ok but that's a logical argument", it's exactly the same logically correct argument as the (illegal) "bases on my limited knowledge, a white/Chinese/male candidate I just met is more likely to be successful at this job than a black/Hispanic/female candidate".