With all the monosodium glutamate and aspartame in today's food (and the lack of magnesium and B vitamins that protect the brain), it is no wonder that people in their 50s, 60s, and 70s are slowing down mentally so much. Their eyesight also changes, sometimes as early as in the 40s. These people should be tested as often as necessary, for health and not for age.
On the other hand, it is possible for someone that is 120 to be able to see clearly, and react quickly. If that person has not ruined their brain, it should remain at a good level of function right up until death. Such people would not have any restrictions on their driving so long as their brain function remains sound.
Now, Jehovah's Witlesses should be examined more often, regardless of age. They do not get enough sleep because of too much field circus, they are under too much stress, and their diets are not conducive to good brain function (plus the crap they are taught at the Kingdumb Hell slows them down even more). Under those circumstances, it is surprising that the witlesses don't get into more accidents than they already do.