we all have our own experiences that we bring from our own childhoods w/ us about this, which makes it even harder to be objective.
While I totally agree w/ other posters that the cut-off dates stink (again, who thought it was a good idea to send 4yo's to K??), I don't think holding back based on chronilogical age is a good idea.
If the child has no true delays (whether academic or emotional/social) warranting holding back, I think it will show a lot that they should be in the higher grade. And kids know it too... that was one of my concerns about holding my oct dd back (which i didn't in the end) - will the other kids tease her that she should be in the older class, or will she feel like "gee mom, you didn't think I could do it???" (that was the main one i always thought about). She's entering 3rd now, and there's a boy in her class that repeated K, and i've heard the other boys (thank goodness not in a mean way) say he should be in the higher class and not theirs. And socially, he doesn't fit in. But he also doesn't read well at all... his parents haven't tested him (who knows why not), but we're all guessing he has a learning disability (and/or seizures... he passed out one morning in the gym - it looked like a seizure, and he says it happens all the time at home)... anyway - poor kid shouldn't be in a private school, because they can't help him as much as a public one would. Whoops... got a little off base... sorry! My point with that boy is, leaving him back in K didn't help him... there's a disability or something there that holding a child back for won't help. Each of my dd's teachers have said this to me at separate times (that holding back won't help a disability). This is why they don't hold back except when it's truly warranted. Her first grade teacher had told me that even if a child fails a grade, they don't automatically hold back. It totally depends on *why* the child failed the grade (that surprised me - I always thought if you fail the grade, you're held back). I'm sure holding back helps certain instances, and I think the teachers can tell that better than anyone.
I remember when my dd was in 1st grade, I would always say to dh "can you imagine her in K now, doing K work?? she'd be bored and feeling like it was baby work and that she was hanging out with babies. Yes, she'd be the *smartest* in the class, but it would have felt silly, like an older kid sitting in K and being *smart*. But this is because there really was no reason to hold her back. Again, if there's a solid reason, definitely talk to the teachers about it, but if not, think about it a lot before you do it. I remember her K teacher saying she and the other 'young ones' took a little longer to remember the morning schedule (hang up coats, put away backpacks, give teacher folder, etc). But the teacher expects this every year and the class is geared for this. But that is *this* teacher... what is your ds's future K teacher like? I would weigh her opinion very heavily about this.
Again, good luck deciding! I know it's very hard!