Interesting story from the OP (I know K'Tesh...hey man!)...but although it describes the elderly gentleman as being confused and forgetful, there isn't one single word describing how this person actually drives, or was driving that night. Confusion and forgetfulness cross over all age groups, given various contributing factors. How the gentleman appeared in the fast food restaurants doesn't clearly indicate lack of fitness to operate a motor vehicle.
In the circumstances of this particular situation, the visual cue of this person being aged allows an association that's easy for some people to make, that this person being confused and forgetful must be primarily due to their advanced age. And maybe that was the reason for the elderly gentleman's confused, forgetful state, or maybe it wasn't.
In the apparent mental state that K'Tesh observed him, if the police officer had stayed to observe the elderly gentleman drive, would the officer have seen driving that was impaired? When an officer sees something out of sorts about the way someone is driving down the road, the characteristics of the driving is what gives the officer the pretext to stop the driver and subject the person driving to a battery of questions and tests to make a determination as to whether impairment is a factor in the driving.
Okay. Try this idea: Without actually having seen the person drive, to check for general impairment, could the police officer have obliged the elderly gentleman to perform the tests that officers request of people that have been stopped for suspicion of impaired driving? Examples of tests I'm thinking of are the 'close eyes, bring your finger tips together', 'close eyes, arms out to sides, one foot in front of the other, heel to toe...'. Simple tests that require a fair amount of mental clarity and physical dexterity to do.
Who is willing to draw up the criteria and tests to determine whether a person like the one observed in the fast food restaurants is qualified to operate a motor vehicle? If the criteria is simply forgetfulness and confusion, start preparing to take from behind the wheel, a lot of scatter brained teenagers, over-worked soccer moms, stressed out office workers, and so on.