For what it's worth, I have heard a number of anecdotes of unschoolers who didn't learn to read until they were 10 or 12, and then became extremely prolific and advanced readers very quickly. Anecdotes are not "real" data, though, and I'm not aware of evidence one way or the other on this.

That said, I'm not a big fan of it the way it was done by others in my extended family. I don't really want to go into the details here, but I feel like there is some wasted potential.