Is it normal to have depression when only 18?

NORMAL- no not?: I think what you mean is whether depression is COMMON in 18 year olds? If you believe you are suffering from DEPRESSION then, it's worth speaking to someone, your parents, siblings, school counselor, psychologist. Often, people mistake CLINICAL DEPRESSION for being sad or reacting to an unfavorable situation. Those types of things do not make a diagnosis of depression. Talk with someone please.

You can have depress: depression at any age, it is not limited to any one group of people, age, culture, religion, sex, religious preference. It is a biological disorder that requires medication or may be due to situational factors.