Ageism is illegal in US under Age Discrimination in Employment Act if that person is 40 or older.

Only if the ageism is on favor of the younger person. It is never illegal ageism to always hire the oldest candidate.

Every ageist situation that I have encountered thus far (am age 46) was "not a cultural fit."

I look younger than I am, but my patience for over-working too many hours is at an end. Years of stress have led to health issues that have to be managed.

But this all rolls into "not a cultural fit."

Yeah, and as a worker in his 20s who prioritizes work-life-balance, I wouldn't be a culture fit at those places either.

Bias was for protected classes was made illegal for a particular reason, which also applies to a less extent to the non-protected classes. To argue that bias is acceptable along as it isn't explicitly illegal is a stance that seems dangerous when applied to other areas of technology, and many past discussion within this community have shown that many members here are concerned with doing more than the bare legal minimum when it comes to ethical behavior.

So the law is wrong. This is the alignment problem, but for laws instead of AI. The law doesn’t perfectly match what society (to the extent that society is an entity with desires) actually wants.