For what reasons might a average weight 20 year old female have high cholesterol?

Genetics: Cholesterol is produced in the liver as well as a product of your diet. Your cholesterol may be high as a result of a poor diet that has yet to lead to obesity due to age, activity and body metabolism. It can also be elevated in someone who eats a low fat diet but has a liver that makes more cholesterol than average. Medications may be needed in that situation to reduce your future risk.

Diet: It is very possible that this healthy, 20 yo female has a diet that encourages a high blood cholesterol level. In addition, the levels may be due to genetics. In either event, she should be examined by her physician to determine the cause and be treated accordingly. This will help prevent health problems in the long run.