Probably because HN users typically

1. are strongly/over-opinionated

2. are power users - most of these design choices are inherently less functional than alternatives

3. understand that these things aren't just "the way they are" but are intentional, explicit things that designers & co. want and will push for over how things used to be as well as the status quo

No, it's the children that are wrong.

I am in my 40s, and so are most of my friends. But I'm dating a younger person (20s) and hang out with his peer group on occasion. So, this is anecdata, but I don't think it has anything to do with any age group specifically.

The modern web and app space is just filled with minor annoyances that individually are probably fine but that add up to a sort of aggregation of frustrations and feelings of being manipulated that is hard to put a singular description to. I get this sentiment in both age groups that I have high exposure to. It's not really about hating new trends because we're just old and comfortable with how things are (or were).

It's even more simple, the people who like a more modern UI don't like (and therefore don't use) Hackernews and its UI

I always figured the HN age group would skew younger not older.