Neuro - China has a history in athletics of trying to hide ages. This past summer, they did it the other way by claiming female gymnasts were older than they actually were so they would be permitted to compete and several former Olympic gymnasts have discussed that they were only 13 or 14 but were claimed to be 16 or 17 to be permitted to compete on an international stage. Whatever is in the best interest of the nation is what is done, and in an effort to achieve a preeminent status in athletics, China will alter ages. To get better gymnasts, they'll lie and say they're older. To make a national star like Yi seem like a better prospect or to allow him to play on junior teams to have a better shot at success, they'll say he's younger.
And as much as you're talking about his early notoriety, there are two flaws there. First, realistically Yi wasn't nationally known as a basketball player at least until he claimed to be about 11 or 12, after he had gone into national sports academies, by which time, when they saw he had potential, they easily could have scraped a couple of years off his age. Yes, His parents were professional handball players, but that doesn't mean they were stars and that anybody paid attention to when they had a child. Second, your argument implies that the Chinese press would have rooted this out long ago. However, the Chinese media is completely and totally controlled by the Chinese government, a frequent topic of discussion and criticism during the past Olympics.
Finally, the proof is only allegations unless somebody says otherwise. But it's not just rumor. It has been reported that his middle school registration listed his birth year at 1984 rather than 1987, which would make him 24 rather than 21.
http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2008/12/22/sports/s020737S38.DTL&type=health