What causes grey hair or at what stage does one starts havin grey hair.?

cause iam 27 and i have grey hair all over.

What causes grey hair or at what stage does one starts havin grey hair.?

According to Wikipedia:

Gray hair color typically occurs naturally as people age, usually turning their hair from its natural color to gray and then to white. More than 40 percent of Americans have some gray hair by their fortieth birthday, but white hairs can appear as early as childhood. The age at which graying begins seems to be almost entirely based on genetics. Sometimes people are born with gray hair because it is passed down genetically. Some people use hair dye to disguise the amount of gray in their hair.

The change in hair color is caused by the gradual decrease of pigmentation that occurs when melanin ceases to be produced in the hair root and new hairs grow in without pigment. Two genes appear to be responsible for the process of graying, Bcl2 and Mitf. The stem cells at the base of hair follicles are responsible for producing melanocytes, the cells that produce and store pigment in hair and skin. The death of the melanocyte stem cells causes the onset of graying.

I don't think that you can do anything to stop it just try to cover it up, you have my sympathy though I started greying at 25, I think the darker you are the sooner it starts.