IMO, it really only is accurate when comparing average children. A bright child who is motivated to do so can certainly learn to read at an early age, but by 8 or 10 that child may not necessarily be any more advanced than the child who didn't learn to read until 7. Dot taught herself how to read at 3. I didn't read until I was nearly 8. Within a year, I was reading on an college level. Dot will be 7 in October, she reads on a high middle-school level (possibly higher, I haven't bothered testing her.)
I believe that the original basis for this whole "they all average out" thing is comparing kids who attended academic preschools (not necessarily "baby university", but just your average basic preschool that places an emphasis on children learning K skills before K) with children who did not do so. Exposure can certainly lead to an accelerated start, but not necessarily one that continues.
I also believe that in some cases very gifted children can have issues that cause their giftedness to not be exhibited academically. My eldest neice, C., is this way. She learned a roundoff/back handspring/tuck combination in less than two months. But she reads on a first grade level. She's 13. When she was 2, I took her ice skating for the first time. She had never been on skates of any kind, much less figure skates. Within 30 minutes, she was skating on one foot both forward and backward. I know kids and adults who still can't do it after months of lessons. Yacko is 14, at 15 mos old he taught himself the alphabet using his wooden blocks. He had NO interest in reading, and didn't learn until kindy. He is not the least bit academically accelerated, but he is mildly gifted, as are Wacko and Dot. Of the three of them, Yacko is actually almost 2 years BEHIND, Wacko in about on level, and Dot is a year accelerated (two if you use our state's cutoff for starting school, since she would have been in kindy last year.)