Why is it that in judo, 10th degree black belt is the highest rank attainable?
I have no way to find out, but I suspect (pure conjecture) that it has to do with the expected time-in-grade for each rank. As I understand it, the time-in-grade in years for each rank is equal to the rank. So it takes 2 years to get to nidan and 3 more years to sandan and 6 years to rokudan, etc...
So, assuming that the practitioner is an adult (mid-20's) when they get their shodan, it will take about 54 years to get to 10th dan. That would put the practitioner near 80 years old (like Karl Geis). Another 11 years for 11th dan would put him in his early 90's (like Helio Gracie), and another 12 would put the practitioner near 105 (like Keiko Fukuda).
Now we have certainly improved life expectancy over the years, and we like to think that with good fitness and healthy habits, more people have the potential to reach those advanced ages, but there is still an effective cap on maximum age - and that glass ceiling is around 115-120 years.
The old dead guys that came up with the ranking system didn't just pull the 10th dan out of a hat and say, "That'll be the highest rank." It's just that not many folks live healthy active lives capable of practicing and/or teaching judo into their 90s and 100s.
This guess of mine also sort of makes the story make more sense about the Judo powers-that-be awarding Dr. Kano 12th dan as the founder to put him "beyond the rank system." It's not that the number 12 is magical, it is just nearly inconceivable that someone could live and practice and teach judo for another 23 years after getting a 10th dan.
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