Any sort of neuropsych evaluation is most likely not objective enough, and could be discriminatory. For example, against autistics, or for them. No need to have that value judgement discussion.
Just have an objective competence exam. Final exam questions from 101 courses dealing with geography, physics, calculus, world and US history, micro and macro economics should suffice. Have the tests written, proctored and graded by a panel of judges appointed by larger public colleges in the country. That should do it. If a 90 year old is still with it enough to get a passing score of say 80% and continue to do so for the next decade, then mazel tov, let them serve.
News: A confused Dianne Feinstein tried to give a speech in the middle of a Senate hearing vote and was told to 'just say aye' instead ( www.businessinsider.com )
"Just say aye," Senate Appropriations Chairwoman Patty Murray repeatedly pleaded to Feinstein during the vote. Eventually, Feinstein did just that....