P.M.
I used to tutor high school kids who had been advanced to the next grade whether they had met the previous year's academic requirements or not.
I didn't get the success stories, and there could be many, but I worked with those who had definitely been harmed by the practice. They had built further academic development on a shakey foundation, or in one case, no foundation at all. (The poor boy was very bright and had somehow managed to fake "reading" when he could scarcely sound out 3-letter words by high school.) They had come to see themselves as hopeless failures, and in some cases, behaved in dysfunctional and distracting ways, based on their low self images.
Kids just starting school don't have issues about when they start (though their parents might). They are generally most happy and confident if they are with a group of roughly the same social and academic level. I think your instincts are good, and I deeply admire the wisdom of parents who are more interested in their children's welfare than the opinions of the neighbors. Go, mommy, go!