Generally, from what I’ve heard in Friend-of-a-friend anecdotes, “not OK” families usually reach a mutual decision to part ways - “If you aren’t going to follow my rules, get out…” As opposed to not-trouble-making but not energetic twenty-somethings which is what I assume is meant by “ok families”.
AFAIK in Canada, most provinces require the non-custodial parent to continue child support (including help paying tuition) for over-18 children who go to post-secondary education; at least up to the first degree. A friend of mine mentioned once her husband was being dinged for money to support his children with his ex, even though he was pretty sure the one was not in college (they lived in another province). It was easier and about as fast to pay for the rest of the year and wait for a college transcript (and then stop if it was not forthcoming) than to go to court to try and prove lack of attendance.
Just as a side note, there are a few cases in the news in past years where children are being made to pay for their parents’ care too. Plus, there was a Quebec case - based on ancient French law - which decided that grandparents had to pay child support for the grandchild, since the father was not around to do so. (But, IIRC, he grandparents had a pretty significant participation in the child’s life anyway).