Most, if not all, martial arts systems have some kind of time requirement for grading to the next belt. At the lower levels (kyu) it might be just a couple of months before the student is allowed to test again.
At black belt (dan) the time between tests is measured in years. A typical time in grade is; 2 years between first and second degree, 3 years between second and third degree, 4 years between third and forth degree, etc. So if a person achieved their 1stdegree black belt at 18 they couldn’t test for their 4thdegree until they were 27.
There are of course exceptions; a person may be promoted because of their contribution to the martial arts, new organizational responsibilities, new teaching assignments, or exceptional talent.
What is the reason for time in grade?
Use the time in grade to explore, polish and sharpen every aspect of your art.