Forty years ago, 96 percent of six-year-old children were enrolled in first grade or above. As of 2005, the figure was just 84 percent. 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.