In answer to the original question:
Think of a child's first 2 or 3 years of life. In that brief time, they learned to walk, talk, and interact with their world in a myriad of ways. No one ever "taught" them. Structured curriculum was not necessary. They learned by observation, trial and error, practice, and a thousand other ways. The parents' role is to model what is possible, provide a safe environment and opportunities, and let the child get on with what comes naturally. Nothing needs to change at some arbitrary age, or for certain kinds of subjects.
Walking and talking are arguably far more complex and difficult than reading, math, or any of the academic subjects. My children, now 17 & 18, are interested in their world, and their education reflects that. BigGirl is less interested in physics than YoungSon, but she knows more about international affairs than he does. Both of them are articulate and generally use proper grammar, although neither of them were ever corrected or "taught" vocabulary.
This is what radical unschooling is for me - a continuation of the natural style of learning each child was born into. Our home is full of books, and we love museums, so we still regularly visit them. But we see it more as recreational than intentionally educational. We have craft supplies around because we like being creative, not for "art class". The Dumplings are nearly grown, and ready to go out on their own soon. But I am confident they will both continue to learn all their lives - through reading, museums, whatever life courses they choose, whether they include college as part of that or not.