just because your child was ready to read at three doesn't mean all kids are. I have worked with kids that could not read at 7 but at 9 were reading high school material. Reading readiness is like anything else. You can't force a child to potty train, do algebra, or read when they are not ready.
but as long as it isn't your child those things don't really matter, do they?
I'm not saying that all children should be reading at 3 just because my daugher is. She is brilliant, of course. But by 5? Yeah, they should already know their letters and at least be ready to work on reading.
And yeah, some kids can't read at 7, but that doesn't mean that we tailor curriculm for the late bloomers or those with learning disabilities. You target curriculm so that it challenges the majority of kids within a 12 month window -- it will always leave some above and below. For those above, you have gifted programs; for those below, you have remedial or special ed programs.
But if you continuously expand that window (through red-shirting) it makes it harder and harder to what is best for the MAJORITY of kids, not the outliers.