your instructor gives you a promotion based on your abilities at that particular period of time.
when you become too old and frail, or very ill and can longer keep up with younger, fitter, stronger, faster individuals, he does not then demote you as punishment - there is no need as the belt was given as recognition of your abilities at a particular time.
also your abilities would be judged in relevant context - e.g. a 21 year old sparring with a 28 year old, age probably isn't much of a factor so doesn't need taken into account so much.
a 21 year old sparring with a 101 year old, age becomes a factor and therefore would be taken into account.
it's very simple to me and i think to most people tbh
bjj is a fighting system.
belts are given based on your ability to use the system well.
nothing to do with how many orphans you've adopted, or whales you've saved, or addictions you've beaten - i think some people are confusing bjj coaches with esther rantzen...