Well...no one's quite sure. The Pittsburgh Pirates say they are confidant in the accuracy of OF prospect Jose Tabata's reported age, but they are also very much aware of the speculation that he may be as many as 3 years older than reported, according to Rob Biertempfel of the Pittsburgh Tribune-Review. Pirates GM was quoted as essentially saying that he is aware of the rumors, but that there is not enough evidence for them to worry.
The difference between 21 and 24 years old may not seem like a lot, but when it's in regard to a player whose value lies primarily in his potential development, those 3 years become extremely relevant. A large part of what makes Tabata such an attractive prospect (and why he was the main piece in return for Xavier Nady in their 2008 trade with the New York Yankees) is the power that most scouts think he will eventually develop. These kinds of developments can be assumed with players who are young for their age (Tabata was in Double-A as a 21 year old in 2009). If Tabata is actually 24, it is less likely that he will continue to develop and his performances at each level look less impressive.