I was just provided a photo of three gentlemen and all three, in one place, were wearing a red obi. I guess they all have mastered their system(s). I am not saying they have not but since, in recent years, I have noted a "lot of red belts" being worn I began to wonder how that could be and what criteria was used to award such a level, grade or rank to "anyone."
I was originally led to believe that there were no ranks above go-dan or fifth level. At least when the dan-i system was created for the Judo world that was the highest level awarded by Judo's founder. This was somewhere around the late eighteen hundreds or early nineteen hundreds.
In karate the Okinawans adopted the dan-i system but I believe no one actually wore the red belts at that time. I have been informed that the founder of the system of Okinawan karate I practice wore a red/white paneled obi, a red belt with gold strip in middle lengthwise or gold belt that seems to be strictly of Okinawan origins (it seems to have fallen out of favor and very few wear them now).
See also: "Did the Okinawans Develop Their Own Belt System?"
It also was my teachings that the only person who wore a red belt or attained such high levels were the founders of a system such as Shimabuku Tatsuo Sensei who created Isshinryu.
So, back to my inquiry, what constitutes a red belt and what criteria says one is a master of a system? I don't know but it seems to me that it should be a combination of things such as, (1) a certain dedication and diligence toward training, practice and teaching; (2) a certain level of knowledge and experience in at least their parent style or system plus; (3) a certain level of holistic understanding of all that is a martial system such as "Theory," "Physiokinetics," "Technique," and "Philosophy"; (4) a certain level of inner peace in the individual; (5) a certain age level such as reaching the winter years of a life long dedication of study, practice and training in their style or system.
I am sure there may be more that denotes one who has achieved such a level of mastery but not necessarily over the more technical teachings ergo why numbers (2) and (3) tend to hold more importance from some perspectives simply because (3) covers the entire holistic training and practice of martial systems at a fundamental level, a foundation that is mastered.
I can say that such master does not come at an early age regardless because only age and a maturity that comes with it can provide a person mastery of this level.
In a nutshell I don't feel that many people can truly wear a red belt even when they create a system in martial arts. I am only saying this is true for me and from my perceptions and perspective. After all, who am I anyway, right?
Lastly, as to technical knowledge I believe a san-dan level should have a solid and complete knowledge and experience in those things where any levels about san-dan are about "polishing and refining" those things. Again, my perception and perspective.