I have to say, once again, as an experienced teacher and as a mother of 2, that you are the best judge of your child . . . but do take all aspects--academic, emotional/social, and physical maturity--into consideration. That is what schools are (or should be) addressing--the whole child! And by all means send them on time--those who choose early or late entry should be the exception. Statewide and Nationwide, I'd venture to say that late entries are the exception. For those seeing an increase in late entries in your neighborhood or social circle, think about how many are trying to send their children early due to daycare expense??? That, nationwide, is more of a trend. Consider where the recent pressure for all-day kindergarten has come from??? Not because 5 year olds don't need a rest or nap!!! (However, in all fairness, there are those holding their child out because they don't want to let go of their baby, too! But, in my experience, that is much more of a middle class phenomenon.) The cut-off date deadlines set by states are a guideline to acheive an average of abilities--but, by all means, you know your child! If you feel strongly about sending your child early or late, there are many options available (public, private, homeschooling, etc.) But I have taught students K - 12, and from my experience, there are often ramifications when students are either very young or very old for their grade. Nothing life or death or insurmountable, mind you, but "things to deal with". (Things as complicated as the ability for abstract reasoning needed for algebra, and things as basic as the onset of puberty/interest in the opposite sex/legal driving age.) The fact that you are interested in this thread shows that you care and will probably do what you need to do for you child.
Good luck in your decision! There are no guartantees in life (or school). Make the best decision that you can and stand by it!
(By the way, my children have begun school "on time" by state guidelines, and are average to above in all areas.)