What age is a person old enough to be diagnosed with borderline personality disorder?

Usually 18, but: Some adolescents can be diagnosed earlier if the behavior patterns have been consistently present for at least a year. Technically though, age 18 is the agreed-upon age because adolescents can be quite changeable before then.

Personality Dx: Personality disorder diagnoses are given if the person reaches age 18 & any age thereafter.

Age 18: The diagnostic criteria require that an individual be at least 18 years old to diagnose a personality disorder. However, we have seen that many individuals who are diagnosed as having borderline personality disorder in adulthood were initially diagnosed with bipolar disorder as juveniles.

BPD: 18 typically, is the cutoff age, but it can be diagnosed earlier, provided symptoms last in excess of a year.