Uplinker
The reason for ageism is, I think, that mangers don't want to employ anyone older than they are.
I saw this when I was in my mid-late 30s, that was around 1988-91. I can only surmise that they think the older person will show them up or want their job. For the most part, I think that we would make them look good! We've climbed the greasy pole before and have found a comfortable balance.
Whether this kind of ageism is more generated by males than females, I do not know.