No one disputes the immediate results of "redshirting," a phrase borrowed from the sports world . Six-year-olds categorically test better  than five-year-olds in kindergarten, and they enjoy greater social and physical maturity that helps them make friends and win at tag. But there's a growing  debate  about the effectiveness of redshirting in the long term—not only for the kids held back, but for their peers, as well.
Some context: Like private school, redshirting is most prevalent among white, Asian, and relatively wealthy families. Here's 2010 data  from the National Center for Education Statistics on the percentages of kids delaying kindergarten:
There are several factors at play here, including the traditional wisdom, backed up by research , that shows little boys to be particularly fidgety in kindergarten. That said, the most striking disparity is also the most worrying. For families earning the least in this country, redshirting is cost-prohibitive  (PDF). As higher-income families delay their kids' kindergarten entry, children from lower-income families end up "competing" against older and more-prepared classmates—at a crucial time for learning and development.
What's more, a growing body of research suggests that redshirted kids might not enjoy benefits over the long run, anyway. A 2007 paper for the National Bureau of Economic Research argues that "contrary to much academic and popular discussion of school entry age—being old relative to one's peers is not beneficial." (Also, unpublished research  from 2012 found that the advantages of redshirting "fade out and appear to reverse by eighth grade.")
Until more-conclusive research emerges, well-meaning parents are likely to continue redshirting their children. And depending on the individual child, that could be the right choice. But as Harvard researcher David Deming says, it's crucial that parents "make a decision with the whole life course in mind."