Each school is different but most WTF schools do not allow a full BB grade until the age of 16. Until then you are considered a Junior BB. As far as retaining senority goes that is up to the school itself so you need to ask. Usually it is based on portfolio, time in rank, and age in that order. As already said, 2.5 years is a very short amount of time to receive a BB grading,especially for a child. Physical talent aside, there are other considerations when promoting Black Belts.