Once I asked to my blog friends, at which age they started to read. I was very surprised that average of those who responded was at 4.
Here is the poll, it is in Latvian, the last row is "don't remember".
http://klab.lv/users/kihelkonna/137911.htmlMy blog friends are between 20 and 45, and they are rather intelligent and successful persons.
Among these 5 which read at 3, there are 2 Phd's, 2 very clever persons (in terms of phylosophy and literacy) but without particular professional success, and one person I don't know.
Among these who learned at 4, all the persons I know are very clever and interesting (because I choose to read their blogs), and also very successful professionally. The same I can say also about persons which started to read at 5; I don't see big difference but may be these "at 3" are more tended to research.
I think, "reading at 3" means also other early learning.