When I was a,child, the school district didn’t have kindergarten. I was still 5 years old when everyone else in class was 6, so I had to play catchup at first, but eventually I did all right. Of course, I had parents who actively encouraged education and teachers who taught instead of indoctrinated.
My point is, if a child doesn’t have support for education in the home and teachers who actually teach, starting children early is not going to make a difference.
We had kindygarden, and it was worthless. Nothing but Rousseauan, progressive, German, Deweyan nonsense. Although I remember they took us to a piazza joint and let us toss our own dough, which was fun. I recall preschool, just barely. It was run by a Lutheran church, which is different. Though we were Catholic, which I can’t explain except to say it was only a couple blocks away from my house.
No real education started until 1st grade, which is when they bothered to teach reading. 2nd grade started times tables, I think, third cursive, and so on. 1st through 7th could probably be taught in a year, if you were really motivated.