What Causes Graying Of Hairs On Beard And Chest?

hello doctor, i request u to pls help me,i am 25 year old. but my beard,Chest & Head Hairs were got white color hairs, i mean only few beard hairs got white color. But this Problem is Increasing Day by Day .I dont know what is the reason behind that.. ? How can i control over this problem and the spreading of white hairs on my beard nd chest ?Can these white hairs reverse in Black Colour Again ?? Pls Help Me