Oh boy, this one hit's home big time! Here's our situation.
DS8 is an Aug child (as was DH) so would have been one of the youngest children in his class. He was/is also extremely bright, but because of this would easily tuck himself in a corner with a good book hoping not to be noticed so he can live in the world of what he's reading. Socially he was shy and not confident even though he always knew the answers, he was the type of child who was more thrilled to know he had the answer right in his own head than to share it with anyone else. Anyhow we decided to hold him back in preschool and start him a year later, for the sole purpose of him building up confidence and not always being a few steps behind his peers as he aged (maturity, driving etc - which was a HUGE issue for my DH). We worked with his preschool teacher to have him become a leader, she had him "help" with lesson plans showing other kids how to write letters, tell time, complete projects etc. HUGE SUCCESS! He went to K, 1st, and currently is completing 2nd, is in the gifted program, is many years above his peers academically (would have been the case regardless of grade), but most importantly to us STILL continues to be a leader amongst his peers, helping classmates with lessons when needed, always thinking outside of the box and is very much adored by his teachers and peers.

No regrets AT ALL!
DS6 is a May child. He was/is very socially outward, total class clown, everyone likes him, he likes everyone, has the biggest heart of anyone I know and will defend & take care of anyone if he notices they need something. Academically he struggles tremendously. He needed to go to K on time, socially he was way too much of a disruption to remain in preschool, I wanted him to repeat K (same school) but the teacher convinced me he needed to go to 1st, socially he would have been a disaster in K again (just way too out there and had the readiness skills for 1st). WORST decision ever, but we sent him to 1st. This year he has struggled, recognizes he isn't at the same level as his peers (academically), acts out more so because of it, or completely zones out in class. Socially he doesn't have any really STRONG friendships, just gets along with everyone, doesn't want to try to answer things in class anymore for the fear of being wrong, and thinks of himself as "stupid" because his brother is "so smart" and he can't manage what he sees his peers doing. 1st thing is we are having him evaluated for ADD since he is pretty much textbook for all symptoms (school has requested this, we think its just who he is

), but we are also retaining him this year so he will repeat 1st. He is actually very excited to have his same teacher again, he feels repeating will now make him one of the "smart kids" and this will give us the next year to not only give him the year to mature some, but if he in fact does have an ADD dx we can work through getting that in check without having a whole new knowledge base to handle as well. The teacher and school staff will also be working with him on boosting his confidence levels similar to what we did with my oldest by letting him take on leadership roles in class (helping the new class transition into routines). We completely regret not holding him back earlier, even if it was 2 years of K... we could very much see he was "younger" but kept thinking his social outwardness would carry him through, but it's clearly not the case.
As a parent you need to trust your gut! I wish I trusted mine last year re my middle DS, but listened to the school who this year completely agree with our decision.