Nbpoirier, I am not sure how going to first grade is even considered an acceleration here since your ds is already spending a 1/2 day with the K's this year? Is it just because of his age?
So far every educator I have asked says it is best for development if he stays with his age mates and just gets subject matter acceleration.
Are the educators saying this about your ds specifically, or just as a generalization? How I feel about it's meaning differs - if they simply have that opinion re acceleration, I'd listen, think it through, then toss it away and go with my own gut feeling. OTOH, if they are seeing something specific in your child that leads them to say this, I'd give it more weight in my decision - and I'd also want to ask specifically what it is re your ds that leads them to this conclusion.
I actually don't think staying with your age peers and using subject acceleration is a bad idea if you can do it - it's what we chose for our ds. We've had some bumps along the road where the acceleration didn't happen (in elementary school) and we've also found overall that acceleration isn't usually all that it "could" be - ds could have learned more faster and quicker than he's had a chance to, and he's found that sitting in his subject accelerated classes can be boring too, because the overall pace of the class isn't accelerated

So that's one thing I'd consider if you're on the fence re accelerating - is your ds, with acceleration, going to have an opportunity to learn at his true pace, or is he simply going to be one year ahead in a classroom that moves slower than his brain moves? That might be a good (best for the situation) solution, but if you're weighing other factors against acceleration, I'd keep that in mind.
It sounds like the concern is often social issues in later years (being the last to hit puberty, last to drive impacting dating and social life, size impacting sports etc). While I can see that side, it is not the only side...
You're right, that's not the only side of the decision. I will tell you though, that two of the families I've known who were eager to grade-accelerate in early elementary found that this type of situation mattered much more to them than they'd predicted once their child was actually in middle school. It's not that they hadn't thought it through earlier - it's just that once their children were actually in middle/high school, and actually around older kids going through puberty, learning how to drive, going out on dates... they (parents) weren't really happy about their children being in the midst of all of that. Please note - this is a 2 family sample - totally statistically meaningless! Just something to think through. It's possible that it might not bother your ds or you that he doesn't drive or date as early as his friends... but it might worry you that he's around all these kids who are moving through those stages. Does that make sense?
Good luck thinking this through -
polarbear