Below is an article that discusses the study in CT. Basically there is a wide age range that parents can choose to start kids in Kindergarten. On avg across CT, 1 in 12 kids start a year later than the earliest they could have. In New Britain (poorer than avg), 1 in 40 start later than could have. In a sample of wealthy towns (Darien, New Canaan, Wilton), 1 in 4 start a year later than they could have. I believe this is just data for public schools also.