I had a child who fell right on the school date cut-off for K enrollment and red-shirting is very prevalent in our district. When I was deliberating the issue, there was only a few bits of advice that I could find:

1) You won't know if you made the right choice until 3rd or 4th grade.
2) If you don't red-shirt, your child will be going to school with many children who are 16 and 17 months older than she is.


The impression I got was that there was no "downside" in holding them back, which knowing what I know now about bright/gifted kids is absolutely ridiculous.

Anytime I hear of someone thinking about red-shirting, I try to remember to send them "What a Child Doesn't Learn".