I honestly don't know what ours is, or if we even have one. However, my sister went to private school for Kindergarten and First Grade because the public school would not let her in. She missed the date by eight days. The date was Sept. 15th, she turned five on Sept. 23rd. She had been going to a pre-school since she was 2 and 1/2 - 3 years old. I am seven years older than her and loved to play school so naturally, she was my student. She was ready. The pre-school teachers told my mom she would be too bored if she stayed another year. When my mom asked our pediatrician about it, he agreed that she was ready to go. It worked out very well for her. She never lagged behind her classmates. The only slack she ever got was that her friends got to drive before she did.
I'm not saying this because I don't agree with you. I've seen many students entered too early (one was in my class last year and we had planned on holding him back but he moved to another school) and it's usually that the parents don't want them home for an additional year. I'm just saying that it all depends on the child.