Schizophrenia is a serious mental illness that interferes with a person’s ability to think clearly, manage emotions, make decisions and relate to others. It is a complex, long-term medical illness. Although schizophrenia can occur at any age, the average age of onset tends to be in the late teens to the early 20s for men, and the late 20s to early 30s for women. It can be difficult to diagnose schizophrenia in teens. This is because the first signs can include a change of friends, a drop in grades, sleep problems, and irritability—common and nonspecific adolescent behavior. Other factors include isolating oneself and withdrawing from others, an increase in unusual thoughts and suspicions, and a family history of psychosis.