What Causes Premature Grey Hairs?

Hi, may I answer your health queries right now ? Please type your query here...I am 26 years old and have recently found some gray hairs growing on my head. From the looks of it, the gray hair has been growing for about 4 or 5 months, around the same time I was diagnosed with a horizontal nystagmus of my eyes.
All members of my family are late to turn gray so why am I starting so young? Is there anything I should talk to my Dr about?