The belt rankings for kids and adults are different. For kids ages 4 to 15, every 20 classes you get a stripe, and after 4 stripes, you are promoted to the next belt. For the adults, they base it more off of skill. Everybody starts as a white belt, but for kids, you go to white yellow, and adults you go from white to blue.
*To be an instructor, you don't have to be a black belt, most of my instructors are purple belts.