Researchers have not identified a definite cause for premature gray hair, but evidence suggests a genetic component, according to WebMD. The definition of premature gray hair varies based on race; Caucasian people are considered to be prematurely gray if it appears before the age of 20, while African Americans can be prematurely gray if hairs appear before the age of 30.
In some cases, health conditions can cause premature gray hair, including thyroid problems and vitamin B-12 deficiencies.
Gray hair occurs because the body no longer produces melanin. This results in a loss of pigment in the hair, which renders it gray, as reported by US News. Most people report a genetic link to premature gray hair; if a person's parent developed gray hair early, his or her chance of premature gray hair increases.