Incidentally, my highschool in Japan had teenage girls who were Sandan and above at Shodo (calligraphy, and I seem to remember one girl being Rokudan) and several teenage boys of Sandan and above in Go (and Sandan in these arts is not to be scoffed at lightly, they have real skill). Tell them you are Shodan, Nidan, or Sandan at something and it won't mean much to them. Indeed, if you proudly state that you are Nidan after 20 years (me), they'll probably think, What a bafoon!
I have a couple of Japanese friends that graduated high school in Japan with a 10-dan in Shodo. The dan system in shodo and such is rather different (as explained to me, in Shodo the first teaching licenses are considered after one reaches 10-dan) and don't carry over directly into many martial arts so your argument doesn't exactly hold up. In my experience the fact is that in Japan today among the general population who doesn't know much about budo the view is more or less the same as in the West "Oh... do you have a black belt?".
That said you are right that in Japan in the martial arts the dan isn't as "strong" as it is in the West and a 20 year nidan is pretty much unheard of these days. I recently spent time with an elderly sensei pushing 90 who is menkyo kaiden in a sword tradition and in his view most of the 8th dans in the kendo renmei were just "intermediate students" and a lot of then had no business teaching yet. He is far from the only voice I have heard express similar opinions of the state of things in Japan and several sensei in a number of different arts have expressed to me that they think the stricter standards over seas are a better reflection of how things should be, where as today in many arts shodan through sandan are basically gimme ranks ("welcome to the club.. here's shodan") and people don't start really running the risk of failing gradings until yondan.
I remember once being part of a conversation between an iai sensei and one of his students where the topic of grading in the kendo renmei in Canada came up (if I recall the student was going to study abroad in Canada soon). The student was up for a grading fairly soon and assumed that grading in Canada might be easier. The sensei quickly cut the student off explaining that the Canadians are much much stricter and if the student was set on getting that grade soon they should do it before they leave because they wouldn't cut the mustard over in Canada. Very strange conversation to listen too as a Westerner who grew up with the idea that Japan works at a much higher standard than we do (true perhaps in the higher grades but still....)
As for getting older, I find that over here, the more you progress the more you focus on "internal" and/or mental things (used in a very wide sense) and less on physical work which seems to carry over very well to aging practitioners. I might suggest that one reason that hasn't happened as much, or at least to the same degree, in the US is that there simply isn't such a group of advanced practitioners to feed off of as an example. The older generation of senior level practitioners and teachers in the US is basically the first generation of people to reach that point and there has been no real example to draw on for what the "norm" might be for people at the level/age, etc.