From a recent NBER paper by David Deming and Susan Dynarski - its abstract:
Forty years ago, 96% of six-year-old children were enrolled in first grade or above. As of 2005, the figure was just 84%. The school attendance rate of six-year-olds has not decreased; rather, they are increasingly likely to be enrolled in kindergarten rather than first grade. This paper documents this historical shift. We show that only about a quarter of the change can be proximately explained by changes in school entry laws; the rest reflects "academic redshirting," the practice of enrolling a child in a grade lower than the one for which he is eligible. We show that the decreased grade attainment of six-year-olds reverberates well beyond the kindergarten classroom. Recent stagnation in the high school and college completion rates of young people is partly explained by their later start in primary school. The relatively late start of boys in primary school explains a small but significant portion of the rising gender gaps in high school graduation and college completion. Increases in the age of legal school entry intensify socioeconomic differences in educational attainment, since lower-income children are at greater risk of dropping out of school when they reach the legal age of school exit.
Here's an ungated version. I know nothing about this literature and, so, won't offer commentary. However, I found this paragraph from the ungated version interesting.
These sex differences can be tracked yet another step, to the completion of a
bachelor’s degree. The sex difference in BA completion of 22-year-olds has been fitfully
rising for over twenty years (bottom panel, Figure 8). Women in this age group are about
eight percentage points more likely than men to hold a B.A. degree, up from two
percentage points in 1984. If we adjust for sex differences in age at first grade entry, today’s difference is attenuated by about two points, or one-third of the growth over this period. Further, the time pattern differs for the adjusted and raw series. In the adjusted series, there is no steady growth in the gap until the late Nineties; until then, growth in the sex gap in BA attainment is an artifact of sex differences in the age of first grade entry. This is a critical distinction for both academic researchers searching for explanations for the gap and policymakers trying to close it. Until quite recently, growth in the sex gap in BA attainment is attributable not to the decisions of adolescents on the cusp of college but rather the decisions of parents and teachers sixteen years earlier.
And for the sports angle:
Redshirting parents appear to believe that relative age matters for children’s
performance. There is no evidence of a lasting benefit to education or earnings from being
older than one’s classmates. There is, however, evidence of a lasting competitive advantage in sports. In Europe and the US, children on elite youth soccer, hockey, swimming and tennis teams are disproportionately born just after the age cutoff for those leagues—that is, they are the oldest of their peers. This early advantage persists, with 60 percent more major league baseball players born in August than in July, mirroring the near-universal age cutoff