I found some white hair in my black hair of head . although that my age is 18 years old .

May be other causes: Premature graying can occur, though it is usually genetic so you may be able to look at your family and see that you inherited it from a family member. There are other causes of hypopigmentation (decreased color) of a hair. Trauma, bacterial or fungal infection, chemical or sun bleaching, or other dermatology conditions can cause this also. Check in with your doctor if you have related symptoms.

Whitening of hair: is usually related to genetic predisposition. Age of initial graying is largely a matter of genetics. However stressors: nutritional deficiencies, chemotherapy, traumatic head injuries, low vitamin B levels, zinc deficiency, severe illness, surgery & marked emotional stress may impact earlier depigmentation in people who are already predisposed to white/ gray hair. Get a med eval to check.