The scalp has a high concentration of sebaceous glands which produce skin oils (sebum) to protect the scalp and hair. However, excessive sebum on the scalp can make hair appear greasy. As sebum production is typically at its peak during puberty, teenagers are particularly prone to developing excessively greasy hair.
Source: http://www.healwithfood.org/greasyhair/#ixzz3qcTzHA7X