Not to mention that it would be almost impossible for an outside agency, like the IOC, to secure evidence that they were underage. More than likely the Chinese government just issued these girls fraudulent birth certificates or other documents "proving" that they are at least 16 years of age. It's not like the Chinese gov't is going to let the IOC go dig through email records to find the communique requesting that fraudulent docs be produced.
Alternatively, their trainers could have artificially prevented the onset of puberty in these girls by training them to keep less than the 10 or 12% body fat required for a girl to have her first period and start puberty proper. If that's the case I weep for those young women who will in all likelihood have serious medical issues and will probably die of cancer before they hit 40.
XI Wiki



