You have to be careful with these studies - it's possible (and the authors of the research article are worried about this) that the later learners had parents who consciously chose to teach them later.
This sort of decision implies a level of involvement in a child's education that might, on its own, lead to better reading comprehension.
This sort of decision implies a level of involvement in a child's education that might, on its own, lead to better reading comprehension.