December 31st cutoff date in my school district on Long Island.
My older dd, now 17, has a November birthday, and was two months shy of her 5th birthday when she started school. At the time I was concerned, but her preschool teacher assured me she was ready for school. The November birthday made her among the youngest in her class, but for the most part her age hasn't been an issue. Her current complaint is that she won't turn 18 until after Election Day, so I guess everything is fine.
The kids in the class of 2008 ...well, there's a wide range of ages. One of dd's friends celebrates her birthday in January, she's almost a full year older than dd. And there's a boy in the class whose birthday is in December, and his parents held him back from kindergarten, so he's closer in age to dd's friend than to dd.
In hindsight, I think it's the individual child's maturity, not their chronilogical age, that made a difference. The handful of kids who were a bit older when they started school had a slight advantage in elementary school, but looking at them in high school...you can't tell who was older and who was younger when they started school.