There is some of that. As a result, many states have increased the enrollment age, and now have transitional kindergarten, or TK, for kids (like my daughter) wh

There is some of that. As a result, many states have increased the enrollment age, and now have transitional kindergarten, or TK, for kids (like my daughter) who would have enrolled in kindergarten at an earlier age a few years ago than they're now eligible for. Seems to me a properly controlled study would have to compare kids who started at an earlier age, then repeated kindergarten, versus their peers who waited a year to enroll. Despite not exactly being feasible, that would measure the effect of the structured environment at a young age without getting so apples-and-oranges.