Hair is a filamentous outgrowth of the skin found only in mammals.
The speed of hair growth is roughly 11 cm/yr.
The average human head has about 100,000 hair follicles.
Older people tend to develop grey hair because the pigment in the hair is lost and the hair becomes colourless.
Grey hair is considered to be a characteristic of normal aging.
The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.
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