Taken from the FAQ section under amateur racers ATV website.
18. If I race “A” (or “B”) locally, can I bump down a class at the Nationals?
No. If you are an “A” rider locally you must enter the “A” class nationally. If you are caught riding out of class you will be disqualified with potential additional disciplinary action.
14. Can a youth rider participate in a higher youth age class?
Yes – but be careful! If you are a youth aged (7-11), you should ride the (7-11) year old class. If you turn 12 before the end of the year and you voluntarily advance to an older age group, you will not be permitted to drop back to the (7-11) class. This applies even for local events. For example, if you are 11 years old and race in the (12-15) year old class at home, you cannot race in the (7-11) at the Nationals. So be very careful before you voluntarily advance an age group before you are required to do so. The AMA recognizes a youth rider's age as of January 1.