Sebum is composed of lipids, or fats. Lipids don't dissolve in water, which is how sebum is able to create a barrier that traps some water in the skin and keeps other water out. Carbon, hydrogen and oxygen are all elements in lipids, and lipids are found in both plant and animal cells [source: Medicine Net].
Sebum contains a variety of different lipids, including cholesterol, glycerides, fatty acids, squalene, and wax and cholesterol esters, but the exact composition of sebum varies with a person's age. For example, a newborn's sebum is very similar to an adult's sebum. However, after about six months, the composition changes. Sebum in a young child contains more cholesterol and less wax and squalene. The composition changes again around the age of eight and then again during puberty [source: New Zealand Dermatological Society].
For information on sebum, see the links on the following page.