Just found this thread and having skim read it I don't particularly have any issue, just found it interesting.
Sure, it should be hard work to get to 10th Dan in any art (if attainable) and generally the guys that do are in their 60's. For example the heads of my Kenpo organisation in the UK and Ireland are, P. Kennedy - 10th Dan (64), J. Birch - 9th Dan (57), A. Fitzgerald - 8th Dan (63).
However, you only grade to 6th Dan in this association and with the minimum time requirements this should take 20 years (from 1st Dan) so essentially if you managed the minimum of approximately 4 years to 1st, it should take you a minimum of 24 years to get to 6th Dan.
Now this isn't always going to be the way, I achieved my 1st Dan relatively quickly (3.5 years), however, due to time out because of work and injury I only achieved my 2nd Dan this year, 6 years after my 1st. It doesn't bother me, but it does mean that I still have 18 years to go to get to the dizzy heights of 6th Dan which will make me 53 and then it is a guess as when I will be deemed worthy to be awarded further rankings.
The reason why I am waffling is that after 6th Dan there is no prerequisite time to your next Dan grade, it is mainly awarded on your contributions to the art, what you have done, how you have promoted it etc.
We have a 6th Dan, J. o'Keefe (SP?) who recently was awarded his 7th Dan and I believe he is only 40. Therefore, with the amount of classes he runs, schools that are affiliated to him and his junior instructors and the amount he does for the art etc (the guy is a work horse and a really nice guy). It is feasible that he could be awarded the remaining grades each year and therefore become a 10th Dan at 43 (however, this is very unlikely as no matter what you do it is still a long hard/fought journey to earn the awarded ranks).
Anyway, my waffle is basically to say that I can see it possible in certain arts for people to attain 10th Dan at an unusually young age.
My 2 pence.