HalJor 
The strongest argument for an upper age limit for the presidency is sitting in office right now. Not for dementia reasons, but so that they live long enough to be prosecuted and punished for their actions.
Wolf in PDXTerm limits and age limits for all our elected officials, in all branches of the Federal government would suit me just fine. I've been saying it for years, to anyone who would listen.
In Congress? You get 12 years, that's it. 3 years to serve to either support or check a President. You make a difference, or you don't. And then someone new takes your place.
Age 65, and you retire. That's it. Special election to cover your seat if in Congress. New judge on the bench. VP takes over, special election to name your replacement.
Over 65? Nope. Not happening. You're out.
The details can be left to people who are better suited for legal things.