The word you're probably looking for is "Racism".
Extreme Racism.
(Curiously not just from the outright Fascists but also from the very people who have spent the last 4 decades doing performative anti-Racism whilst destroying Democracy by making it a secondary power to Money)
The Nazi way of looking at people never went away, they just changed the lists of ubermenschen and untermenschen - the lives of those from the sub-human races are clearly worth much much less than the lives of those from the master races (which is curiously reflected on how most of the Press will talk about Israelis getting "murdered" whilst Palestinians merely "die").
(For me Germany is especially disappointing in this regard - a nation supporting a SECOND Holocaust, is clearly not better than when it did the first)
I belonged to a small left-wing party in my own country where the oldies (the party was born from the union of parties which dated back to before the anti-Fascist Revolution in my country, which was more than 50 years ago) kinda decided to pass the baton to the younger generation and almost as one stepped aside and passed control to mainly 20-something years olds, all of whom scions of the Middle Class.
The result was that the party went up in votes when the Left had a resurgence, got into an informal coalition with the mainstream supposedly (but not really) center-left party in Government, to keep the right-wing one out of power, and after that in a period of two elections collapsed to the lowest vote ever.
As I see it, the party leadership lacked experience (being all from a narrow social circle and lacking all both broad and long life experience) and didn't even have a concrete pre-made Ideology to guide them like, say, the old-fashioned Communists or even the Neoliberals have, because they dropped the strength of ideology of the old guard when they took over and instead just made it up as they went with no strong anchoring on fundamental Principles or a well thought framework, so ended up copying stuff they saw on the Net from Anglo-Saxon countries which was basically right-wing shit disguised as left-wing (i.e. for example "Equality" by treating people differently depending of the genes they were born with, aka Identity Politics), were easily manipulated and outsmarted by the mainstream party leadership and wouldn't really spot social or economic problems until they had hit their narrow socio-economic segment for a while, or have well-thought strategies to solve them, ending up being just another bunch of politicians making the same promises as the rest (which means that outside the party faithful they had trouble gaining trust from the electorate who just saw them as people who sounded the same as the rest.
IMHO, I think left-wing movements need variety both on the ages of the people involved and their backgrounds in socio-economic terms and life experience, to avoid falling into political traps, becoming little more than groupthink circle-jerks and being disconnected from most of society - being left-wing means working for the many rather than the few, so you should probably have people from all over and of all kinds, in positions within the movement were they're actually participating in setting the direction of that movement, rather than having only a narrow age range, socio-economic background or path in life in control.
The fetishization of youth is narrow-minded and self-defeating for a left-wing party, especially nowadays, when most young people grew up under Neoliberalism thus have interiorised as "the way things are" many economic and social practices they grew up with which really aren't "the only way to do it" just the way things have been done in the last 5 decades, and have never thought about Politics in Power Dynamics terms since, unlike in the old days, there is very little talk of non-Governmental forms of Power in present day politics.
This is just as true for the fetishization of old age, social class or educational level - it's not a specific age range, socio-economic origin or path in life that's the problem, it's the narrowing of ideas, perspectives and sources of information that is the result of one group monopolizing the discussion and decision-making.