greying of hairs at an early age is common these days...and this can be due to various reasons..like.. heredity...that means if any of the parents have had premature grey hairs then children are likely to get the same trend. other causes are stress smoking thyroid disorder vit B12 deficiency certain genetic disorders actually greying can be corrected to some extent if treatment is done before the age of 20. after this age its difficult to reverse the greying but further greying can be delayed.