My opinion is that in the general case people over 70 shouldn’t be driving and I say this as someone who has 4 spritly grandparents in their 90s. I really don’t like how dangerous roads are, a fact that we accept because we did not really have good alternatives, now that we do we should implement them.
You have no way of knowing that. There's no reason it should be written into law. If they can pass the test, then they can drive. Testing already takes care of what you want. If it truly isn't a temporary condition, then you have nothing to worry about.
All cognitive decline is not equal.
If they're able to drive they should be allowed to