You cannot achieve a full black belt until you are 16 years of age, this is why there is a junior black belt rank. There are often physical limitations before 16 years old that would not be realistic for a child student, no matter how skilled, to face an adult aggressor. There is a saying in Judo that given two opponents of equal skill, the larger opponent will more often win. Granted, more often, is not always, but there are facts in nature that are very difficult to overcome. The test for junior black belt and adult black belt is the same, age alone discriminates at this point. On your 16th birthday, a ceremony can be held honoring you in the final step of reaching your black belt. If you are 16 at the time of your black belt test, then junior black belt is skipped.
First and Second Degree Black belts in Ju-Jutsu are called Okuiri (Entrance to the Secrets). This is a great descriptor for this grade and puts things into perspective. You are not "finished" by achieving your black belt. There is a lot more to learn but you are at the point where you have proven you have a firm foundation to continue to build your house upon.
The black belt test is very fluid and flowing. Stamina, determination, heart, and all skills taught are up for display and critique. The only formal parts of the test are kata (including Tai Sabakis) and formal throws. Shisochin is a new kata that is added and practiced for black belt.
Minimum Time in Grade: 6 months