According to the National Center for Education Statistics, some 6% of kindergartners are redshirted nationally. But the numbers can vary by neighborhood. Data from Connecticut's department of education show the incidence of redshirting ranging from 2% in poorer school districts to 27% in wealthier ones. Redshirting is easier for families that can afford an extra year of child care or preschool tuition—and the practice can be controversial because of the perceived advantage that it gives to such children.

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The research on the benefits of being older is mixed. Elizabeth Dhuey, an economist at the University of Toronto Scarborough, didn't specifically study redshirting, but she has published several studies showing that being relatively older in a class has some advantages. In one large-scale study, Prof. Dhuey and co-author Kelly Bedard compared the birth months and test scores of more than 200,000 students in several countries. They found the oldest students in fourth grade scored 4% to 12% higher than the youngest, a trend that continued in eighth grade. In another large-scale study, Prof. Dhuey and economist Stephen Lipscomb found the relatively oldest students were 4% to 11% more likely to hold leadership positions in high school.

Many researchers say that studies on redshirting show no long-term advantage, with any early benefits fading by the time of middle school. As one researcher put it: If you're redshirting as a way to get your child into Harvard, you should rethink your strategy.

Princeton neuroscientist Samuel Wang, co-author of the book "Welcome to Your Brain," says that being around more mature peers actually benefits younger classmates, both behaviorally and academically. He points to a large study that found schooling influences intelligence more than age: The youngest children in a grade scored higher on IQ tests than children the same age one grade lower.