There's no way to prove any of this. The stuff found on the internet from some Chinese website is even more suspect than if it had been from some other website because of the strict control the Chinese government has over internet access in China. People asking for an investigation seem to forget that China is a communist country. They don't provide free access to any sort of investigation, especially anything that can implicate the government. They're not going to be happy with anything the Chinese government provides and there's no such thing as damning proof when it comes to verifying age. Might as well accept it and move on.
There's an funny article up at Yahoo's Olympic gymnastics page about how the parents of the Chinese girls are indignant that nobody believes that their girls are 16. The article suggests that perhaps there's a racial element involved, that asian girls are naturally smaller than caucasian girls. There is some truth to that, plus non-asian people have a difficult time guessing the ages of asian people. I'm Korean and 27 years old, but I routinely get mistaken for a high school kid. Asian people are generally better at guessing ages of asian people, but even to me a few of the girls on the Chinese team look younger than 16. Indignant, yea right.
The cake is a lie.