Im in gratified's camp -- it took me a while to realize what was going on!
I consider ds6 was "a reader" at about 3.5yo, though he was reading before that -- that was the point where he could choose a picture book off the shelf at a bookstore and read it. He was a big sight reader and didn't bother with phonics-- he extrapolated phonics from his sight word knowledge rather than vice-versa, so he had a ton of sight words and new words almost automatically became sight words. This really speeded his reading and comprehension. He tested mid-second grade at 4.5yo, and mid-fifth grade at 5.5yo.
So I guess I consider a "reader" to be a child who can pick up a new book, read it fluently and understand what he's reading.
