But yeah, I wonder how many of the younger US posters (regardless of race or family income) are residential, 4-year college students, or set on that track if they’re still in high school.
The majority do not do that. And remember that what the political commentator class and what the upper 10-25% of Americans regard as the "normal" college experience is basically the experience that you only get if you go to a fairly selective university.
Most people aren't taking liberal arts or humanities classes beyond 100-level requirements that are basically just a repeat of high school. Most people do not do a senior thesis or anything resembling meaningful "research" during college; the closest they get is the occasional 10 page paper that they mostly Wikipedia'd their way through.
I'm thinking of the college experiences of some of my extended family who are close to me in age and they include:
1. Went to mediocre public school (Texas State University) for a year and then left because of poor grades and "not knowing what he wanted to major in." He took classes at community college part-time the next year because his parents made him but then he gave that up and they didn't force the issue. He was working as a waiter at that time and after he left college he eventually became the general manager of a restaurant (no idea what he's up to now what with COVID going on).
2. Went to mediocre public school (University of Texas-San Antonio) for a year and, as above, left because of poor grades and general lack of direction. She worked at a locally based retail chain and then moved over to their corporate office to be an office assistant. Then they basically told her they liked her and she was doing well but if she wanted to get promoted any further she had to have a college degree because reasons. So she went back to school part-time (University of Houston) to get a degree in human resources management, finishing when she was about 30 or so. Now she's a human resources manager.
Basically, that's a closer approximation to what college is actually like for most people than someone going to, say, the University of Texas-Austin, majoring in philosophy/political science, being a research assistant, doing an internship at Brookings, writing a senior thesis and graduating Phi Beta Kappa.
That is what college is like for the sort of people who write about politics on Twitter and in mainstream media.