Most likely you have entered the wrong age when you registered your Google Account. Unfortunately, since you have done that, there is absolutely no way to change it, that is the limitation of having a Google Account. You would need to create a new one. [1]

Thanks,

Mohamed Mansour

[1] - http://www.google.com/support/forum/p/youtube/thread?tid=0694272557c18113&hl=en