Earlier quoted context omitted.
"Ageism seems largely concentrated on the Silicon-Valley style society." I saw ageism many years ago at a big aerospace firm in the L.A. area. It was understood and even outspoken that young engineers would get the call to work extra hours and weekends if needed, as well as the occasional graveyard shift when a unit was in thermovac testing. Older engineers were not usually called upon become they had "paid their due…
That's not ageism; that's a gentle form of hazing, and to be found in almost any profession or trade. An extreme example is what medical students go through in the early years of their study; a mild example is how, when I worked nominally as a cashier in a grocery store as a high school kid, the more experienced folks ran me from pillar to post for my first couple weeks' worth of shifts, doing everything from stockin…
Also, while mentoring is important part of seniors work, it is not "own time and effort". It is paid on the clock time and effort. I am not mentoring overtime for sure.
Hazing is not rational. Its main benefit is that incumbents feel good about power trip they are allowed to do. Plus a bit of good feeling after someone else has to go through uncomfortable thing you yourself was through in the past. Nothing more, nothing less.