Why do some people who are young get gray hair?

Different heredity. Hair greyness is a bell shaped curve distribution and some are on the early side of this curve and get grey hair at a young age. It is not due to diet or lifestyle.
Genetics. Graying of hair is related to genetic predisposition. The age of initial graying is largely a matter of genetics. However stressors such as nutritional deficiencies, traumatic head injuries, low vitamin b levels, zinc deficiency, severe illness, surgery and marked emotional stress may impact earlier depigmentation in people who are already predisposed to gray hair. In general – it's genetic !