A personality disorder can be diagnosed by having a mental health professional looking at and analyzing long-term patterns of functioning and symptoms. Diagnosis is typically made in individuals over the age of 18. Because people under the age of 18 are still developing their personalities, they are typically not diagnosed with personality disorders. Some people with personality disorders may not recognize a problem, as it may only be apparent to others. In addition to this, it is possible for a person to have more than one personality disorder. It is estimated that around 9 percent of American adults have at least one personality disorder.