I read that WaPo article, and it seems to me that a big part of the problem described there is that employers want people to put up with stress and overwork and not just put up with it but act enthusiastic about it. Employers may predict that older people are less likely to go along with oppressive working conditions. I know I don't. Resistance to inhumane demands is a benefit that older workers can provide to the younger workers, and that's just what employers may prefer not to have around.
I think it's important to make a distinction between ageism coming from employers and ageism coming from younger people in general or younger employees in a workplace. I'm saying this not just because it's the age discrimination by employers that violates the statutory law, but because the reasons for the ageism are quite different.
Employers are hoping to extract more and better work from the people they hire. Younger people may dislike older people for any number or reasons, some of which are more pernicious than others, but they may also derive benefits from older people, at least some of the older people.
I'm suggesting that one of the benefits of older people in the workplace is our longer perspective and our ability to perceive and willingness to object to poor working conditions.