All function declines with advanced age, though genetics, accidents, environment, and behaviors all play a role, so there is not one clock by which every individual can be measured. Still, there is a limit to both functional acceptability, and statistical probability that might be reasonable to apply to someone who purports to be capable of “running the country.” I know a 98-year-old mathematician still doing world-class abstract mathematics, but it would be foolish to bet he will still be able to do that four years from now, especially noting that he gets winded after walking 150 feet. And even if the cognitive cylinders are all still firing what about the executive functions--the ability stay focused for extended periods, to plan, organize and exercise self-control? I think a decent candidate should volunteer to take tests that show high passing marks for all of these mental capacities. It is not probably reasonable, however, to forbid a candidate who is statistically unlikely to continue to be robust through four more years. Actuaries have determined someone who reaches the age of 90, on average, will live another four years, but I don’t know when, statistically, our decay outweighs the expectation of robustness. A panel of physicians could examine an individual and pronounce on his or her fitness, but it would be better for We the People to weigh the evidence and vote accordingly, each to our own conscience, and be thankful that even if we make an egregious mistake it probably won’t last more than four years. One hopes less, because impeachment is always possible.