Most kids are a lot more "ready" than the parents think. In my experience it is usually more about the parent not being ready than the child. The parent decides that the child needs another year and finds a justification for it. Given the chance to go to school on time, it is very rare that a child is actually held back. The vast majority of the kids that parents were convinced were "not ready" do just fine. In our public school system 90-95% of children start K on time and the retention rate is less than 3%. It is even less for those same children in first grade. About half the children in public schools here don't come from homes where anyone really cares wether they are ready or not, and makes no effort to see that they are. They have had no preschool, and have not been taught much if anything at home. If there were truly as many "not ready" children out there as parents seem to think, the retention rate for these kids would be a lot higher. The retention rate is even less in private schools for children who start at the correct chronological age. Why not give our chidlren the opportunity to succeed, rather than holding them back until we are ready? Yes it is hard and scary as a parent to let go, but that is what we have to do as responsible parent.
There has to be a line drawn somewhere, or what you end up with is 7 year olds in K whose parents chose to hold them back to "give them the best possible start". What they really wanted was a child who was stronger, faster, and smarter than everyone else, or they didn't want their "baby" to grow up. The parents of the kids in DD's class CHOSE to hold thier kids back so that they would be 7 in K. There are 2 of them from 2 different families, but the story is the same. They were not held back because they were not capable of doing the work. It has backfired on them as far as the social aspect goes because they are so much older and don't relate well to the kids in K along with the fact that they are really bored and have become a discipline problem because of it. Thes kids SHOULD be in first grade.
My point is that an age cutoff has been set and I feel it should be respected unless there truly is a valid developmental reason not to, not just some vague "my kid isn't ready" excuse to get around the rule, but a diagnosed condition that makes waiting a better option. You may say "what is a few months". The next Mom will say "if she gets to wait 3 months I'll wait 6. My kid will have the advantage then." The next mom comes along and decides to wait a year so that her kid is top dog. It has to stop somewhere. I say everyone should respect the age cutoffs until there is evidence that a child is not functioning well on that grade level, or that they have a disability that prevents them from doing so. It would eliminate many of these issuse of "old kindergarteners" being 6 and 7 and "young kindergarteners" being 4 and 5. Holding a child back for no more reason than you want to ultimately creates a set of classroom problems that takes away from the education of every child in the room, including the one who was held back.