There are no age minimums or maximums in order to test/compete at the USFSA's novice, junior and senior levels.
You can only "skate one level up" in U.S. non-qualifying competitions and not at qualifying competitions like Regionals and Sectionals.
Eligibility for Junior Worlds is based on AGE ONLY (click on the link I provided above for a more detailed explanation) -- that's why, for example, Beatrisa Liang has been able to compete at the past 2 Junior Worlds even though she has competed at the senior level at U.S. Nationals since 2001. Liang is technically age eligible for Junior Worlds through 2007 (when she's 18).