In the case of marijuana legalization [...] ISTM that most people above a certain age are against it and most people below a certain age (except politically right-wing ones) are in favour of it [...]

A common error people make when they see an old vs. young split in opinion is assuming that it must be an age effect rather than a cohort effect. Thank you for avoiding that mistake by noticing that it could be either! (Maybe I could use that as a rationality litmus test, ha ha.)

I can think of no social, geographical, or political factor which would substantially correlate with belief in astrology, like, at all. (Maybe I just lack imagination, though.)

You got me curious! I pulled up a chapter from the NSE's latest Science & Engineering Indicators report; it links a spreadsheet of survey results from 1979 to 2010 on how scientific US adults think astrology is. (Strictly this isn't the same thing as believing in astrology but I'd expect it to be a fair proxy.) In the 2010 sample, people who were young, female, less educated, or knew fewer science facts were more likely to think astrology was scientific.

I should say that this doesn't automatically mean astrology is a worse rationality test than atheism. Atheism itself correlates with sex, race, age, and education level, at least in the US.