The belt ranking system, as it is now, originated in Japan. The founder of Judo, Dr. Jigero Kano, adapted it (from Japanese swimming) to differentiate between the various levels of his pupils. As now comprised, the belt system runs from 10th Kyu (class) to 1st; and, then, from 1st Dan (rank) to 10th. It must be noted that there are variations in the numbers of classes and ranks in different styles. Some Jujutsu styles have provisions for up to 12th (Junidan) Dan.
Though standards may vary in the Western World, with its emphasis on accomplishment, in the countries of origin, Dan rank is indicative of acceptance of an individual into the group (association) while the lower classes are for those aspiring or apprenticing to join the group. With this in mind, it is easy to understand the differences in standards for rank achievement among the various styles of martial arts. Loyalty, many times, is a major consideration for Dan rank. Ryukyu Kenpo Kobujutsu provides an excellent example of the differences in the requirements for Shodan. A Ryukyu Kobujutsu student has no reason to learn much of the basics taught to a Ryukyu Kenpo student; as all of his training focuses on weapons' training.
A very new development, centering mainly in the commercialized West is Junior black belt rank. That rank is given to very young children, generally, as a result of their fulfilling the rather limited requirements of that particular organization; and, to keep Mama and Papa happy and paying.
Traditional styles maintain age limits. Ryukyu Kenpo Kobujutsu for example limits Shodan to age 16, 4th Dan to age 22, 6th Dan to age 30. A 10th Dan in Ryukyu Kenpo Kobujutsu is over 60 years old and has been a black belt for over 45 years. Other systems have their standards, but; it is very rare and unusual for someone under 45 to legitimately obtain 8th Dan or higher.
Before the kyu/dan ranking system came into being students were given various documents attesting to their proficiency. The one thing a black belt is not, is an instructor. A black belt is not a license to teach nor does it imply the ability to teach. Instructors are given a separate document testifying to their ability to transmit the knowledge to the next generation of disciples.
An Instructor is a special kind of black belt and is awarded teaching licenses in addition to his black belt certificate. The best instructors (sensei or sifu) will be neat and clean. Their attitude will be friendly, professional, and respectful. Not only to you, but, towards their students, other instructors, schools, and styles. They are forthcoming about their credentials: when did he start studying? Who was his instructor? When did he reach black belt? How long has he taught? Did he create this style or is he part of a well established system? What association does he belong to? Is his teaching certificate from his association current and up to date? (Completing a governmental agency coaches' course or its like, does not mean much). Who is the association head and what are his credentials? Can the instructor promote you or must he bring in someone else to test you?
Someone who has "created" his own style must be approached with caution. There are legitimate reasons for expanding one's knowledge and horizons. An instructor may wish to expand areas in which his style is deficient. But, to learn one or two or three or more different styles to black belt level and then combine them to form a single "new" style, is a joke. To palm it off as legitimate, grossly unfair to the unsuspecting students who are receiving nothing solid or substantial and no hope of advancing beyond intermediate level.
The kind of student an instructor turns out says more than anything else. A competent, traditional instructor will produce students who are polite, respectful, well mannered and helpful. Capable of independent thought and defending themselves, these self-reliant individuals are a true testament as to his abilities.