My grandfather is 82, and until recently he had a full head of hair but now is getting thinner

Senile alopecia (a very bad name) is what you are talking about. I believe that every hair follicle on the body (scalp, beard and body hair in general) has  known life span. When it reaches this, a condition we doctors call Apoptosis (cell death), the hair may very well fade thinner and thinner over time. The good news is that you have to live a long time to see it happen. That is, of course, better than the alternative.