Not sure if this has been coverered, but I wrote something recently on this topic and thought I would share some data on redshirting. Nationwide, about 5% of kids are redshirted, but it varies dramatically by district, with some districts having as many as 20% of kids redshirted. Most redshirted kids are male, white, and wealthy. Date show that at age 4, children who later were redshirted were no different academically or socially than children who were not. While many poor/working class/middle class parents are also worried about their kids' academic progress, they simply can't afford to redshirt, so they don't.

The belief among academics is that redshirting is a social/peer pressure phenomenon of the upper middle class that also is a response to fear about increased K expectations.