I would agree with your list with one critical addition... kids primarily only socialize with kids +/- about one year of age, which is critical because all their friends know, understand, and probably like the same idiotic clothing/music/media/tech fads they like, simply because they're the same age and grew up together in the same location. Kids are strictly uni-cultural. There is just The Culture (apologies to ian banks).

On the other hand adult social areas like HN cover from lower teens to retired and are international (even though HN is mostly SV)

So when I was a HS senior, 100% of my friends knew about, and most of them liked, the same idiotic music and clothing I liked. On the other hand, on HN only a tiny fraction of the audience would even recognize certain band names I thought were cool.

The lack of individuality of kids has enormous social media implications. Inherently there are going to be social media fads which work very well among identical interchangeable kids that are simply impossible for multicultural adults to understand.

Not just "short space of time small radius" but also nearly identical age/maturity level and very similar socioeconomic class and basically mono-cultural.

In other words high school and college are pretty much the opposite of the real world, which is not exactly news, but...