The researchers state:
Though older students have an early edge based on an extra year of skill development, the study maintains that older and younger students learn at the same pace once they enter school, based on a review of federal education data.
The study found, for example, that older kindergartners scored 24 percentage points higher than younger peers on standardized reading tests, but the gap narrowed to less than 4 percentage points by eighth grade.
“Kids learn at lot before kindergarten, especially if they’re in preschool. One way to think about it is that the oldest kid in kindergarten has about 20 percent more life experience,” Lubotsky said. “But once they start, they basically learn at the same rate.”
Of course, parents should also base their decisions on their child's behavior and social development.
Lubotsky says parents still need to weigh children’s needs and consider holding them back if they are immature, can’t sit still in class or have other issues that could affect learning. But he says the study’s bottom line is that kids are generally best served by starting school as early as possible.
In a perfect world, which we know doesn't happen in most schools, each child would be evaluated on their own abilities and not their age.