The problem is that it's China's secret to give away. It would be the only way to be caught (as in the case of the underage gymnast in 2000). Every/any publication coming out of China is govt controlled. Any official document could have been created and planted by the Chinese govt. It can even be the case that some of the birthdays were never officially documented and just made up later. If I were to write a research paper, I would spend part of my paper on the involvement of the Chinese govt in the sport, and on the media; just to allow for the possibility that official birth certificates/ passports cannot be used as a determining factor in the case.
While the "baby teeth" theory is a good one, also remember that in many places, it's quite common to pull damaged teeth without replacing it with an implant, or getting a crown.