In the end cut off dates really don't matter because no matter what month you use as a cut off, someone is going to be the youngest. Case in point. A few years ago RI had a cut off date of Dec 30. So a lot of people chose to "red shirt" or "give the gift of time" to their Oct, Nov, and Dec. babies, (especially boys), leaving boys born in July, August, and September as the youngest in the class.
DD has an October birthday- she was 4 going into school- In second grade there were 5 kids in her class with October birthdays, but 3 of them (2 boys, one girl) turned 8 while DD and another boy turned 7. Were they really peers? Who knows, but they were all in the same class.
It got a bit dicey at Christmas time in third grade when she had a boy who was 9 and a few months away from turning 10 telling all the kids in the class that Santa wasn't real. No I was not ready for my just turned 8 year old to give up the magic of Christmas, but that is what can happen when ages in classrooms become inflated.
Now the cutoff is Sept. 1. So what happens? Kids (especially boys) with June, July and August birthdays are being kept out. So that made my DS whose birthday is the end of March one of the youngest boys in his class, and the year he entered Kindergarten he was 5 1/2 going in. (Which didn't bother me BTW). (I know of a few instances of kids(boys) born in April/May who were not sent to school the year they turned 5- that means they turned 7 while still in kindergarten.)
To the OP just from reading what you wrote, you should not send your child to school because it sounds to me like you've made up your mind that he isn't ready and doesn't know enough. If he doesn't do well, you will second guess yourself for his entire school career. Keep him home and be done with it.
But to everyone who red shirts, or gives the gift of time to their kids for whatever reason, I don't want to hear how smart...mature... or how well behaved your kid is... he SHOULD be.. after all he is a whole year older than some of the other kids, and it's really not fair to compare him to kids who went to school when they were supposed to.
But a lot of people do it.