Although this isn't the usual use of the term, your comment is a great example of the importance of a manager that values diversity. You can overlook, and drive away, really good employees for really bad reasons. It's not likely that a response to a random slack in the middle of the night had any value to the company's bottom line. Managers that can't accept that not everyone should be like them are bad for the company.

You highlight another good point, in that ageism is just a symptom of a broader psychological phenomena of "othering" others. I suspect that even if I was indeed younger than my manager, simply having a child and entering a new life stage with newer, more important priorities caused me to be treated differently. In a way I think the manager just wanted people who are just like them because they were a lazy manager, and wanted an easy way to relate to others rather than the hard work of understanding other perspectives and experiences. A ton of companies I have worked for preached diversity of thought, opinion, and background, to the wazoo and very rarely have they practiced what they preached.

This is real diversity--some single, some married, some with kids, some without, some older, some younger, coming from all different educational backgrounds, and living in different communities.

If everybody went to the same handful of colleges, is roughly the same age, and lives in the same trendy suburb, it doesn't matter what color they are. That "diversity" is literally skin-deep.