No one is saying that they are exempt. But there is a reasonable floor where most people have "fully matured" to whatever level they personally are going to.

Nobody at 23 has the life experience and humility to run an entire town effectively.

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An upper ceiling is a lot iffier for sure. I think in general we should try to avoid very old people who may be vulnerable to medical concerns (though mayor is kind lower stakes than president or governor) but I don't think I would straight up legally prohibit it.