You can't revoke someone's license just because they're old. Some old people drive well. You have to renew your license every few years anyway. They only require a vision test. They could require a complete driving test for drivers passed a certain age.

From my experiences, older drivers are the ones who go five mph under the speed limit, and younger drivers are the ones who cut me off and (from the perspective of my mirror) drive partially inside my trunk. Why not require a brief mental agility test for everyone during regular checkups at the doctor's office where the results are forwarded to the DMV and license retesting is done based on these results?

I don't agree with this. If you test proficiency, this doesn't matter.

If you're concerned that a quick decline in health would make the driver unsafe, possibly we should look into a paragraph in the wording that says after a quick decline, a driver should self-deny his own driving until he can be retested or pass a prescribed self examination.

Pilot's medical certificates have this stipulation.