Can my junior athlete race up with older juniors?

Let's start by reviewing the rules: 

1H10. Mixing of Classes, Categories, and Genders 

(d) Under 23 and Junior riders may compete in any races for their age or older, subject to category restrictions but not Masters races.

(e) Youth riders may enter only youth races. Youth races may include only riders from age 6 through 8.

(f) In mountain bike endurance races, junior riders age 14 and younger who ride with older Juniors may race up at the discretion of the race administration, but generally should enter races for riders up to category 2.

*For USA Cycling National Championships juniors will need to race their age group and cannot race up with older juniors.

It is always good to check with the race organizer ahead of the event about signing your junior up with older juniors.