I just ran across this 2010 article and thought others may find it of interest. A New Zealand study showed that children who were taught to read starting at age 7 showed no differences in reading skills by age 11 to those who were taught starting at age 5. The study compared children being taught at an older age at Steiner schools with ones being taught at the conventional age of 5 in the state run schools.
We unschoolers have often observed that late readers catch up quickly. Though this study didn't address truly late readers, it makes sense that a child who learned to read at age 8 or 9 would have a similar experience if their late reading is not due to their having a learning disability.