The warning sign idea is idiotic, and would be illegal, as it would be blatant age discrimination.

However, I do feel they should require more frequent driver's tests for drivers over 80. Like every year. While it's true that elderly people rarely drive recklessly, there are many cases of elderly people accidentally plowing into crowds of people (or storefronts) because they are senile and have no business behind the wheel. Other elderly people are still good drivers, so this should be determined on a case-by-case basis at the DMV.

There should also be a program where a direct relative can report an elderly driver to the DMV as being unfit, and they would then be immediately tested. Many senile and/or stubborn people over 80 don't want to give up their license despite being unfit to drive, and there's nothing any concerned relatives can do about it.