Personality disorder is a mental disorder that affects a person's pattern of thinking from paranoia, antisocial, narcissistic, to borderline, dependent, histrionic, etc. Anyone affected by it is just “different from other people” because of the distress that it causes those affected by it and makes functioning difficult even in simple things. For example, one has to be 18 years to be diagnosed with antisocial personality disorder even though the patterns of behavior like manipulation that qualifies for its diagnosis may start to manifest by age 15.