I think we as a society just don't believe enough in the learning capacity of adults. I've recently taught my sister (24, ex-chef) all my highschool math (i was in a reinforced math class) plus analysis 101 in the span of two weekends. It was good enough that she now is in the better half or her college math class, full of 18 years old who just finished HS (and most of them with reinforced math options). My best friend learned spanish in less than three month at 27.
A corollary to this observation (adults, or a least young adults, can learn really fast, and even faster than adolescents) is that we need more formation options for adults. You should be able to leave school at 14, work a trade, then stop in your 20s to integrate college and learn something new. And college could be a more open place with ex-plumber, ex-cooks and ex-electricians in it. The best software engineer my age i worked with worked on HT lines for 3 years.