Given the apparent standards of contemporary academia, it would be my guess that most home schooled 12-year-olds are already schooled beyond the college level.
My son is a freshman at one of the colleges recommended by ISI this year and he says he is well prepared, having read most of the materials that are required reading for Western Culture and Rhetoric already in homeschool.
"How long before we have college level home schooling?"
It's already here. My daughter is planning to take at least two years homeschooling for college.