How early should one send a child to school. Too early, and she may miss critical time with parents and not be ready for school. Too late, she may have to deal with classmates that are not as fast or as developed (see previous post on redshirting). But it is not just a question for an individual, one needs also to figure when as a society school should start. France, for example, offers public kindergarten very early (2.5 to 3 years old), and it much more pedagogical than daycare. Most French kids enter first grade with reading skills. In the US, kindergarten is at age of sex or even seven, and there is little teaching. China is similar.
Dionissi Aliprantis uses the Early Childhood Longitudinal Study and exploits cross-state and cross-time differences in the cut-off date that makes children eligible for kindergarten as five-year olds. Children were analyzed in kindergarten, 3rd, 5th and 8th grade. It turns out that an earlier cut-off date is better, and an earlier birthday as well, as long as the child remains eligible. But the analysis only pertains to rather small start age differences, so it is difficult to extrapolate to even earlier start date as seen elsewhere.