Ageism is real. It's not some sour grapes of under-performers who let their skills decline, or move or think at a dotard pace. It's essentially "We don't want anyone old because old = bad, new = good."
Maybe they were not under-performers 10-15 or 20 years ago, but if someone gives 20 pages of resume and most of it happened in 90's I would not be interested. Because it is quite easy to see such person is hanging onto his past performance like Al Bundy to his 4 touchdowns in high school.
If someone would be 50yrs old or even 60 but his last couple of years are taking most of resume and they did interesting work at that age I would be curious to talk to such person.
It is not that they would need to have latest frameworks or libraries listed on the CV. It is more about if they did meaningful work in last year and not that they did meaningful work 20 years ago and now they are just hanging around doing whatever.
But then under performers are interviewing more so they cast bad impression on others that are their age/ethnicity whatever.
Just like the saying good developers are not on the market for long and one that make good impression are hired quickly.