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.
Darien is great because they have more kids playing than any other program in the area. They focus on development over winning at the youth level. Because they have been so good people that are crazy about Lacrosse (like anyone on this website) choose to move to Darien over other towns in the greater NYC area. They may have some older kids but I would attribute their success to them running an incredible youth program.