I mean it was a hyperbolic example, the point being that they often drive much slower than the posted limit and/or the speed of traffic, have problems merging, accelerate extremely slowly, brake unpredictably, or are confused. Of course the same could be said about any driver, but it's just a basic fact of physiology that human faculties decline with age and that operating a heavy motor vehicle at that time can be hazardous, especially with the size of vehicles today.