I don't know if I have any advice, but thought I'd share my own experiences (if it helps, I don't know!) My birthday is in October and our school cut-off in NY was December 1. I started kindergarten at age 4, turning 5 in October. I had no issues whatsoever academically or socially. Actually, I sort of enjoyed being the "young one", and I don't really know why. That ended in college when all of my good friends turned 21. I wasn't 21 until October of my senior year! Ended up getting in trouble for a fake ID because I just HAD to go out with my friends. But anyway....
My oldest daughter has an end of August birthday. Our cut-off is September 30th. She is by far the youngest kid in the 5th grade. One of her closest friends actually turned 11 in July, before my daughter turned 10 in August. Do I regret sending her? Not necessarily. She is gifted (was selected to go to a different school for the Advanced Academic Program - we chose to keep her at our neighborhood school, but that's a different story altogether) School has always been easy for her and I don't see it changing. Who know, maybe it will? I do notice that she is not quite as socially mature as some of the other 5th graders. But she has gravitated towards other kids like her and has lots of really nice friends. She is not interested in some of the things that her peers are (boys, looking good, mean girl type behavior) She is just herself and she's confident about it. If she lacked confidence I might worry. But she's happy and doing well, so for now all is good.
Daughter #2 is a November baby, and she missed the school cut-off by 5 weeks. She is one of the oldest in 2nd grade (she is 8) What I've noticed is that she is much more mature than her peers, and can't stand goofy behavior, kids not paying attention or doing the right thing in school. I really don't know if this has anything to do with her age or it's just her personality. I'm definitely beginning to see more leadership qualities in her. And I'm happy she is where she is.
Anyway, it truly is a tough call. I'm sort of a believer of sending a child to school when she is eligible, but only YOU can really know what is best for your own kid. I hope I don't end up regretting sending my daughter at age 5. It will be interesting to watch my two - one being the youngest in her class, one being the oldest. Sort of a social experiment - kidding! Good luck in your decision.