Basically any and all of the degenerative diseases that afflict the elderly can impart their driving. Arthritis, failing eyesight, etc. Not to mention alzheimer's. My grandfather would repeat himself because he couldn't remember what he had just said, kept telling me to study hard so I could get into college(I was 28 and had already graduated) but still had a license to drive. My Father and Uncle finally had to sit him down and demand that he give up his license. He fought like hell too.