The term red-shirting around my area doesn't really have anything to do with kids close on the border or near the age level. It's used for kids who are far beyond the age level who have been held back, primarily for sports reasons.
That's pretty much the group of older kids that wind up with that label (redshirted) here, too. However, it seems to be more for academic reasons than sports reasons where I live. The oldest kids are often the ones who are identified as gifted by teachers and tracked accordingly very early on.
We have a 5 by 9/15 or 5 by 10/1 cut-off depending on which district you are in locally. Dd#2 has a 9/27 bd & started in the 10/1 district then moved to the other. So, she is naturally the youngest in the class. She started K about a month before her 5th bd. There are kids in her current class who turned 7 by the middle of kindergarten. One of these kids who comes to mind is recognized as really bright, is being subject accelerated for math and goes to the GT reading class. Maybe she is gifted or maybe she should just be a 5th grader (and not even a younger end 5th grader) rather than a 4th grader this year.
My kids have bds right at the cusp where we could have waited a year to start them. I don't know that it would have given them an unfair advantage to do so b/c they wouldn't have been outrageously older than their grade peers had we waited. However, IMHO there should be a really, really good reason to wait to start your child in kindergarten until s/he is nearly 7 and not just he's shy or you want him to be the best athlete b/c he'll hit puberty first.
Like others have said, if it is a choice that is made by a small # of families due to personal circumstances, it really isn't an issue that tends to get the rest of us all riled up. However, when it becomes the status quo and your child is then 1-2 yrs younger than the rest of the class b/c so many kids were held out that extra year, normal developmentally appropriate behavior gets pathologized. And kids who are gifted but also a lot younger tend to get overlooked for enrichment in favor of kids who are achieving highly b/c they are a lot older.
