Canities, It may be premature if it's below 20-25 yrs in both the genders, it may be due, oxidative stress, genetic causes, vitamin deficiency, smoking,.etc Since your 25 yrs at least you can avoid further progression of greying  by using these Medicines, Altris 5%spray lotion on the scalp skin on the affected areas daily night for minimum 6 months, Can use tab.kerablak tablet daily once after food for 3months. Use gentle cleansing shampoo like Hair 4 U shampoo daily or alternative days head bath.