That's highly subjective and no doubt any assessment is likely to strongly align with partisanship.

Which is why we need age limits. You can't expect people to objectively evaluate whether those inside or outside their party are still legislating or judging effectively in their 80s. Obviously most Republicans would lean towards arguing that she's senile and most Democrats would lean toward the opposite.

And then I imagine cognitive decline does not affect all faculties equally - so someone may still, say, have no trouble writing essays, but their judgement may be compromised.