The idea of a health test keeps getting tossed out a lot as a solution, but it has the issue of all qualification tests for office -- the loss of perceived legitimacy of representatives if it was seen as a biased tool that's open to manipulation at worst, or functionally useless by being too narrow at best. I can't think of another trait besides maximum age to use that would be seen as fair, and good luck getting any constitutional amendment done in our political climate right now.