My son is 13 years old and is having the problem of premature hair greying. this is not the age of whitening of hair so what could be the reason ?

See Your Doctor: Premature gray hair in children is uncommon. Can occur in otherwise healthy child if pigment production stops, resulting in graying similar to adults. Diseases that can cause graying are: Vitiligo, Genetic disorders, Autoimmune & Thyroid Disorders, Anemia (Vit B-12 Deficiency), & more. Treatment consists of hair dyes & styles to minimize effects. See your doctor for correct diagnosis & treatment.

Non-genetic causes : of premature graying include smoking, Vitamin B 12 deficiency from a vegan diet, tuberculosis, & hyperthyroidism. Genetic causes found on Chromosomal Microarray are either benign or have other smptoms; e.g., absence of intrinsic factor needed to absorb B12 causes pernicious anemia; Waardenburg Syndrome has deafness. A 3-generation family history, physical exam & lab studies are needed.