Some parents purposely hold their children back a year before entering kindergarten. See the MSNBC article about it: http://www.msnbc.msn.com/id/38993761/from/toolbar
Kindergarten dilemma: Hold kids back a year to get ahead?
I have always wondered about it issue. Some people may look to research, but allowing parents be the judge may be the best approach in this case. Although I do not have any children yet, if I had a child who fell into the "younger" range for their grade level, I would consider this possibility. Depending on the geographic location and school district rules, some parents are forced to hold their children back.