My mother held my youngest brother back from Kindergarten (he was born one day before the cutoff date). She felt that unlike his older siblings, he was not ready. I don't think that decision hurt him one bit and if he was ever bugged at school for being older or having 'flunked' he would rightfully have said that his mom had made a decision and there was nothing that he could have done about it.

For a different perspective I would add this: I was a special education teacher for 32 years. Most years we had a preponderance of fall birth-dates... a fairly consistent correlation. 

The central message is this: every kid is different. Some are ready at a certain time and others are not. If a child has not spent time in a larger peer group (eg. day care) then it's difficult for any parent to know how their child's abilities compare to those of other kid. Day care providers and ECEs have a better sense of those relative measures/indicators because they see kids interacting in group settings.