http://www.newyorker.com/online/blogs/elements/2013/09/youngest-kid-smartest-kid.html
Youngest Kid, Smartest Kid?
by MARIA KONNIKOVA
SEPTEMBER 19, 2013
The New Yorker

Quote
While earlier studies have argued that redshirted children do better both socially and academically—citing data on school evaluations, leadership positions, and test scores—more recent analyses suggest that the opposite may well be the case: the youngest kids, who barely make the age cutoff but are enrolled anyway, ultimately end up on top—not their older classmates. When a group of economists followed Norwegian children born between 1962 and 1988, until the youngest turned eighteen, in 2006, they found that, at age eighteen, children who started school a year later had I.Q. scores that were significantly lower than their younger counterparts. Their earnings also suffered: through age thirty, men who started school later earned less. A separate study, of the entire Swedish population born between 1935 and 1984, came to a similar conclusion: in the course of the life of a typical Swede, starting school later translated to reduced over-all earnings. In a 2008 study at Harvard University, researchers found that, within the U.S., increased rates of redshirting were leading to equally worrisome patterns. The delayed age of entry, the authors argued, resulted in academic stagnation: it decreased completion rates for both high-school and college students, increased the gender gap in graduation rates (men fell behind women), and intensified socioeconomic differences.
The article has links to the studies it cites.