ImKamii - you and many people under the age of 18 have been having this same issue. Unfortunately it cannot be resolved.
EA has employed landmark computer algorithms that are usually used by larger marketing firms. Basically they run a quick background on your Gamertag, and match it up with any other data found on the internet that could be related to it (e-mail, Facebook, ect) and will assign an age based on this data. Even if you lie and say you're 46, the EA super-software will find your correct age to 97% accuracy. Activision (Call of Duty) is currently gearing up to use this same tech.
Can't cheat the system. See you in 3 years.