What makes a 10th dan?
While judo 10th dans are all supremely skilled at judo, the difference between a 10th dan and a 6th dan is not technique nor judo skill, but rather contribution to the art. Consider these excerpts from a list of profiles of judan-ranked judoka:
He was unbelievably energetic and eventually stood at the head of the Kodokan's instructors. The speed with which he mastered the techniques of Judo can only be matched by the rapidity of his promotion... He was a permanent member of the Kodokan Dojo Consultative Group. He is the author of the Canon of Judo.
As a young man he was very keen to go abroad but in 1906 was asked to become Judo instructor at Keio (the oldest private University in Japan) and he remained there for more than fifty years, devoting his whole life to the work. He was a member of both the Kodokan Council and the Dojo Consultative Group.
In 1899 he became the head of the Judo Section of the Butokukai and later traveled extensively teaching at schools and police establishments. In 1931 he began teaching at the Kodokan and was a member of the Dojo Consultative Group.
All his efforts were poured into the training of young teachers and he was of the greatest assistance to the President of the Kodokan. He did much to gain for the Kodokan the secure position it enjoys today
The Shihan's assistant from the very founding of the Kodokan, he is one of the great names in Judo. He entered the Kodokan in 1884 and gained a formidable reputation from his contests with the strong men of other jujutsu schools during the Meiji period. He later went to theUnited States where he taught Judo to President Theodore Roosevelt
he worked for many years spreading Judo and training new teachers.
Author of Kodokan Judo: Throwing Techniques, and several other judo textbooks. He has been the chief instructor at the Kodokan for many years, manager of the Japanese Judo Team at the 1976 and 1984 Olympics, an international referee, and he won the All Japan Judo Tournaments in 1951 and 1954.
The repeated themes are not competitive success, though that is a factor and certainly important. Nor is it technical mastery, though all of them possess that too. It is devoting one's life to judo: working to further the art, training new students and instructors, moving in order to establish a far-away school, representing judo in government.
In the end, the ranks above 6th dan are for spreading the art, political success within the ranking bodies, past competitive success, and sharing the highest level of technical expertise with the next generation of students. So no, the 70-year old 10th dan on the brink of death who sits in a chair to teach class is not going to beat a 26-year-old 4th dan who recently medaled at the Olympics. That's not what the 10th dan is awarded for.
High-ranked eldery judoka in competition
Elite competitive judo is a young man's game. The training necessary to maintain the razor's edge of conditioning and reflex is severe, and not suitable for the elderly. That's not to say that old men can't be competitive and best their younger counterparts, but merely that there is a reason that judoka in their 40s don't win the Olympics in judo. However, it is interesting to note some of the age-outliers, as in this 36-year-old:
One of his impressive competitions was the May 1926 Emperor's Cup final facing one of the young upcoming strong players, Ushijima Tatsukuma, a 26 year old 5th Dan. He won a decision here after a hard competition to take the title.
To understand this, watch 10th dan Kyuzo Mifune's Essence of Judo, particularly the parts where he plays randori. Randori is meant to be as vigorous as possible, but one does not smash weaker players unless they are training for competition. Mifune's opponents are attempting techniques but not attempting to overpower Mifune. If they did, he would be in danger of injury--and they in danger of injury and social backlash.