No one knows exactly what makes hair go white. Malnutrition, severe illness or shocks have resulted in cases of white hair, sometimes within a few days or weeks.
Heredity plays an important part.
You can see people in their 20s who have white hairs mixed with their normal hair. It is usually traceable to parents or grandparents who have had a similar pattern of hair colour change. The hair in these cases is usually coarse or medium coarse in texture. Old age is the most frequent cause. As we grow older, many parts of the body economise and slow down.