Read the book. It explains why.
They have found that starting kids at 4-5 does not provide any educational development advantage over waiting until the child’s nervous system is mylenated.
They have compared the kids started early and the kids started later and found that by age 10 or so, they are at the same level. So you can force kids to endure school for several years as little kids or wait until they arem more mature and spend only a year or two for them to end up at the same place. Meanwhile, one group of kids has lost their childhood of palying and exploring and learning life’s practical skills by being forced into tedious book learning.
Nor did I say not to teach them anything. It’s formal book learning that doesn’t need to be started so young because of the nervous system maturation.
And some kids will learn to read early anyways. Parents are certainly more than capable of teaching basic life skills if they simeply spend time with them and reading to them.
What book is this from?
So now it’s formal learning, not education. OK.
As far as keeping up with natural growth and development, language acquisition is before age 8 and 10. It’s very powerful at age 3 and 4, when they sit on their parent’s lap.
Psychomotor development is later, to be sure, but not for all. Gifted students need challenges sooner than the norm.
And for those who like a competetive edge, time is sparse. The advantage is to those who get there sooner.