> It takes people twelve years to learn to read and write at a 12th grade level.
Nah. You are mixing up the skill 'writing glyphs on a piece of paper and retrieving that information via optical means' with 'being well-grounded in a wide array of subjects so you can express an idea in a way that is mentally stimulating to a potential reader'.
Neither takes 12 years to learn. The first, we teach within a year, at most. The second we do not teach at all, and the best 'writers' often are those who were challenged outside of school, by parents who gave them a rich, intellectually interesting environment, not within it.
We most definitely teach number 2. That's what English class is - you read books, plays, maybe documents, and then you write essays on them. You need to be concise, persuasive, have a unique voice.
Just because you, and other's, did not pay attention or were not good writers, does not mean we don't teach that. We do, for years.
There is no such thing as a 12th grade level. 6th grade is all the farther English is taught. There are sometimes things called levels above that, but they are not levels in English they are levels in some specific specialization and have more to do with that specialization than reading. (doctors would not be expected to understand deep computer science writing even though doctors have very advanced reading skills in a medical domain).
I understand Spanish only goes up to a 5th grade level while Japanese to 9th. This is a reflection on how complex the writing systems are, and not the kids, intelligence, or school systems.
The years after you have your basic reading down are used to learn other skills that are of general use in life for everyone.
It takes people twelve years to learn to read and write at a 12th grade level.
You could study it for another 4 or more years at university if you wanted to develop your skills further.
> What made me good at reading was reading books outside of school.
This is your personal experience. I learned to read and write quite well at school and was well-prepared for university.