Several years ago I taught two semesters of remedial English classes for freshmen at the University of South Carolina. The students had not met admissions requirements but were accepted anyway. I took them on, expecting to meet kids who still didn't know the difference between a subject and an object, who specialized in run-on sentences.
I've got to say I liked them; they were just plain fun, full of surprises the nonconformists, the cutups, the pranksters, with a few borderline juvenile delinquents thrown in. But when it came to composition, they were in far worse trouble than I had imagined.
They didn't know how to think logically. They didn't know how to think sequentially. And that's why they were poor writers. They were not stupid. They were untrained. And they had made it to 18, meaning sloppy thinking was habitual.