My son was born at 32 weeks and had a 30 day NICU stay. We have been very lucky. He has never been behind developmentally, although always small for his age. He should have been born 12/26, but came on 10/30, so if he would have been born anywhere near his due date he would have missed the K cut off date. He is currently 5 1/2, 43 inches and about 40lbs. We made the decision to keep him at his preschool and enroll him in the Kinder program last year. He just 'graduated' last week and after watching his the past nine months, we know we made the right decision for him to 'give him the gift of another year'.
He is currently reading at 1st grade level, knows shapes beyond the usual circle, square, diamond, etc., uses a ruler, understands simple multiplication problems...the list could go on and on. Academically he could go right into 1st grade. He grew tremendously socially, is more mature, and very considerate. I can't imagine him being in 1st grade this year, attending with kids that are 8-10 months older than him.
My husband is a teacher and was also born in October but was not held back. We have a lot of insight when it comes to teaching and how kids learn, but deciding what to do was still a hard decision. At this time, we do not regret our decision one bit. We live in CA and it would have been difficult to hold him back for a second year in K, but if we decided that we wanted him to go to 1st grade in the fall it would have not been a problem. We couldn't hold him back later in his life (say in 6th grade) and did not want to regret not giving him that extra year. The only obstacle was having to decide to pay for it, and as of right now we have no regrets about spending the money. We know that we gave him an extra year of growth in many areas, not just academically.
We know our local elementary K program very well and know it would not challenge him, so because of his success he is going to a 100% Spanish Immersion school in the fall. He would have been with kids who haven't been to preschool; who can't recognize colors, letters or numbers, spell their name, etc., and would have probably got lost (ignored to some extent) in the classroom because he is 'advanced' for entering Kinder. He has had some Spanish at his previous preschool, so we know he enjoys learning this language and figure if he doesn't like it or isn't successful we can move him back to our local school for 1st grade. To ensure he doesn't lose his reading abilities, we are continuing his current reading program over the summer and into the new school year where he will only be taught and spoken to in Spanish.
I can also tell you that in our experience, boys are a given to hold back and it is a toss up for girls depending on the child. As the other posters have said, every kid is different and you never know how they will succeed in whatever they do. I can tell you that we have had several friends in this situation and there has only been one who has regretted not holding their child back (all the others did...girls and boys). We get all kinds of comments (opinions) on what we should have done, the 'real' reasons why we are holding him back, and some comments that are just rude. So beware that not everyone will be on board with your decision, no matter what you decide, but know that it is your decision to do what is best for your child.
Sorry this was so long. We have been in this process for almost two years, so there has been a lot of discussion!