Guess I'm going to disagree with you there. I don't think that being post-pubescent makes you an adult. Teenagers I've known (including pretty much every HG+ one I've ever met, and including people with IQs north of 160) have been a long way from being grownups.
Being able to read classic books or talk about politics doesn't make you a grownup. Being a grownup comes from having a body of life experiences that you've learned from, and simple arithmetic keeps this out of reach for 14-17-year-olds who live at home, haven't had babies, haven't had to pay bills every month, buy supplies for themselves, find housing, or get something done repeatedly without adult supervision, etc. etc. etc. Plus, there is also the issue of brain development, which continues into the 20s. Most 18 year-olds aren't really adults in the true sense of the word, though they're old enough to start feeling their way by that point.
In a four-year residential college, many of the students living in dorms, especially those fully subsidized by their parents, are not adults by several of the criteria you mentioned, yet colleges make little effort to regulate their behavior, and it is impossible for parents to do so. I don't think there should be a supervision "cliff" at the time students enter college. What the slope should look like is hard to say.
