Oh, we had a lot of experience with this topic! Our son's birthday is in the last few days of August, with a September cutoff. It never occurred to me to hold him back, and his preschool teacher didn't recommend holding him back. Then I found out that EVERY boy at his school--through even March & April--was staying back. The school director showed me the birth dates of the kids going into the 2 kindgergarten classes, and the closest boy in age to him was going to be 6 months older than him, and the oldest would have been 18 months. He was a preemie (he was due to be born in mid-October), but healthy. He is/was gifted, and was showing all the signs, but I wasn't really picking up on them yet, despite things his teachers were saying (I thought they were just being "nice"). HIs pediatrician convinced me to hold him back, saying that he was really due in mid-October, in which case I wouldn't have been allowed to send him to K. I ended up falling for the "there's no downside to staying back and doing the pre-K class", so that's what we did.
K was fine; he loved school and his friends. Then in October of 1st grade, his teacher and the enrichment teacher called us in and wanted him to skip to 2nd grade. This is not common in our school district--he's the only kid who had skipped a grade in the past 5 years--but they really thought it was for the best. It was stressful to do it mid-year (it didn't happen until Thanksgiving time), and we worried about what other kids would say, since it was a pretty obvious switch. But it was totally fine. And I was kicking myself for technically "redshirting" him in the first place--we could have avoided this whole situation.
Fast forward to 7th grade. Now I have mixed feelings. Yes, he has continued to excel academically, and is going to be going to the high school for math next year in 8th grade, so it has not been a problem academically. On the other hand, I don't think it has been a huge benefit academically--the next grade really isn't all that much harder, so once you make the adjustment, for truly gifted kids, school is STILL way too easy, and skipping one grade doesn't solve that. Even skipping 2 grades doesn't turn regular school work into a challenge. Fortunately, he is a polite but social child, so I try to appreciate a lot of the social elements of school, and know that he is more challenged by things like Science Olympiad, MathCounts team, his writing assignments, reading exceptional literature, etc.
But there IS a downside to skipping ahead, and to me it rears itself in middle school. Puberty, etc. Fortunately, he is a taller kid, and there is SUCH a range of sizes in 7th grade, that he doesn't look out of place physically. But I do think he will be a later bloomer puberty-wise, and the things I am hearing that the kids are into absolutely SHOCK me. I really have no wish to have him spending all of his time with boys who are 12-18 months older than he is--I'd prefer to let him be a little kid a little bit longer. So while I wasn't concerned during elementary school, now I have some regrets about him being the youngest.
But SOMEBODY has to be the youngest!!!! And logically, if the August people keep their kids back, then the Julys will want to, then the Junes, then the Mays....until you have 9 year olds going to K.
My wish: that just like we can't choose to skip our kids into K early, I wish you couldn't hold them back unless there has been some sort of evaluation that determines they need to be kept back (obviously the medical issue mentioned above would qualify). I also agree that if you redshirt for no other reason than to "give your kid a leg up", then they shouldn't be in g/t programs in K, 1st, or 2nd grade--if they are academically ready, they should go to school! I never would have tried keeping him back if all those other summer & spring people didn't do it. I don't mind him being the youngest, but I just wish it was within a 12 month band, with some other boys his age, or near his age, in his grade.
I admit, I'd rather see ability grouping happen than skipping grades--my son would learn so much more in a high-ability 6th grade class than in a heterogenous 7th grade class. I have not found skipping to be a fantastic solution :-(
