The hair is an appendage of the skin. It is composed of the same kinds of cells as are found in the cuter layer of the skin, which is known as the epidermis. Hair grows from hair follicles which are deep recesses in the epidermis.

The sebaceous glands of the scalp secrete an oily substance called sebum, which is the source of nutrition, luster, and blackness of the hair.

The hair can not be fed externally, for such nourishments, as the scalp requires, must come to it from the bloodstream.

Why do some people go grey early ? And does it have any health implications ?
Hair grows in stages, with a certain percentage of hair either growing or resting at any one time. The hair follicles the little bags under the skin where hair is formed out of skin .... also contain pigment called melanin.

When the body stops producing pigments, the hair become colourless, turning white. Mixed with darker hair, this produces a look most commonly associated with "grey." An actual grey hair may be a result of pigment dilution.

Links to Other Health Conditions.
Premature graying has been associated with certain medical condition such as thyroid disorders, vertigo, vitamin B12 deficiency, and anemia. Vertigo is an autoimmune condition in which the cells that pigment (melanocytes) are destroyed, resulting in patches of hair and skin that become white.

There may also be a link with decreases bone mineral density in post-menopausal women, which is a risk factor.