It's so far unclear whether extra school in middle adolescence benefits or harms those affected—some studies find a benefit to cognitive or non-cognitive skills, others don't. Some find benefits to earnings. These are all affected by the usual problems: issues with identification, lack of controls, fade-out, and publication bias. But the evidence on earlier schooling is much less divided—and it almost universally finds that going to school too early stunts child development.
What's more, "too early" is well within the range of when we currently send kids to school. In Britain kids go to school at four or five. But a Danish study (pdf) found that even at around age seven starting school later led to less crime and delinquency through life. This study—and most of the others I present—used a "quasi-random" study design.
For example, the authors might use arbitrary cutoffs. If someone is born on 31st August and another person on 1st September it's likely that a jump in some variable between them that isn't seen between 30th & 31st August birthdays, or between 1st & 2nd September birthdays, is down to the effects of the cutoff.
In Brazil, starting school later made kids more likely to get into university. In Germany, starting school later made kids less likely to smoke, and healthier throughout life. In Louisiana later starting was also associated with lower crime, especially for disadvantaged groups in high crime areas. In Israel later school boosted maths & Hebrew performance. In Finland it boosted average educational attainment. In Australia it cuts obesity. And here's a second Danish paper, this time linking later starting with lower hyperactivity and inattention.
Now I can't claim any expertise in child development. In fact, I'm almost completely ignorant. So take this as a conjecture rather than an explanation for these findings. But here is a very interesting paper (pdf) from Aaron Blaisdell that offers a possible reason why school has such consistently bad outcomes for kids when applied too early: stunting child development.
Children love to play. Why do they find such a frivolous activity so pleasurable and desirable? Perhaps it is not frivolous, but instead is an adaptation designed to guide proper cognitive development in human children.
To understand why, I marshal evidence from different fields to build a case for play as a central behavioral mechanism of human brain and cognitive development. I start with a discussion of human evolution, focusing on the evolution of human physiology, tool-use, the human brain, and life-history strategy, and development, and how these are all connected as an adaptive suite.
The anthropological and developmental evidence suggests the existence of an extended childhood adapted to establish the skills, knowledge, and understanding necessary to become a successful hunter-gatherer. I also compare human and chimpanzee brain development, and how brain-specific genes evolved uniquely in humans to foster human brain development.
I conclude with the evidence from developmental psychology that even contemporary, first-world children are born with the drive to learn and develop intellectually through play. In this framework, human play can be viewed as an adaptation that guides human brain development to produce curious, intelligent and well-adjusted adults. I close by speculating on the possibility that barriers to or constraints on play may hamper intellectual and cognitive development.
I focus on the important concept of developmental decanlization as a mechanism of evolutionary mismatch. I argue that more empirical study is needed to better understand the importance of play compared to other forms of education for optimal intellectual and cognitive development.