What is the difference between black belt and purple then? What I've heard is that the purple belt has the fundamentals and is qualified to teach but just needs more mat time
On the other hand, everyone loses actual ability (as opposed to knowledge as they age). Should say Helio have been demoted to say a blue belt (or even a white belt) as he got into his 90's (the point at which I'm assuming he'd be so frail even most beginners could easily beat him)?
This is a problem inherent in belt ranks -- what do they mean? If it means actual ability to perform, then they'd be more like title belts in MMA and boxing -- something you have for awhile and then relingquish.
Kano (who started the belt rank concept in martial arts) thought of them like school grades (no surprise, he was a university professor), which meant that once you passed a grade it was yours for life, even if you couldn't pass the test years later (I suspect I'd fail many of my undergraduate tests if I suddenly had to write them again, and even probably some high school tests). For him it was knowledge based, and that's still the model judo uses.
The Gracies seem to have followed that model too (though with the belt colors corresponding to different levels than judo's) -- you don't lose belt ranks in BJJ as you lose the ability to perform (or so I gather).
A performance based rank would end up being like say wrestling or boxing, where your rank is your current competitive placing (Olympic or national or regional or high school etc). There's a lot to be said for that, I think it works better in many ways than the knowledge ranks in judo and BJJ.