To try to add something of value to this conversation (good luck right?) Why is it you do not see this shihan level of skill in younger people? We see many amazing artists in the MMA world with great skills in their 20's and 30's, yet it seems the great examples of non sport arts are always 50+. Is this a positive or negative trait of traditional arts like aikido?
How do you know you don't ?
I think it's quite possible to have some younger people with amazing skills in aikido that you will never hear of. There is no television spotlight on traditional arts like aikido. There is no monetary payoff in the traditional world. Quite frankly, there is also the fact that it is a numbers game like anything else. You need to have a lot of raw recruits to find those with the raw physical and mental talents to allow the technical syllabus really shine. And you need to find them early enough (in their teens).
Find those willing to train hard for a future that offers no fame, no TV exposure, and most likely no money. I had a very talented teenager as a student, great physical shape, smart, picked up technical skill quickly and rose in rank fast, what might have been possible in 10 years ? However, he also was a top performer in BMX racing - money, fame, tv, graduated HS. and he had to make a decision ...
Even so there are those that have and do exist. Take for example someone like Bruce Bookman who started training with Yamada Sensei when he was 12 or Chuck Clark's son Aaron Clark who is quite good in his late 20's high ranked and now in his 30's. There is no TV spotlight on this or local promotion machines. In you aikido, you have to age, achieve really high rank, have an organization newsletter or magazine talk about you, hold large seminars, etc. to you achieve the kind of visibility that would make you aware of them. There is no promotional venue to point you out in your 20's or audience to watch you.