Why do you want a diagnosis? Technically, professionals aren't supposed to diagnose someone under 18 with a personality disorder. That's because symptoms can be temporary in teens and children.

Personally, I think the "disorder" begins before 18. However, no doctor or psych that I know of would give that diagnosis to a 14 year old. Does it matter?

I don't think it does. The DSM V has a new way of diagnosing PDs. It has to do with rating the severity of different traits rather than just classifying. One of the reasons for this is to help direct treatment. Treatment is geared towards behaviors and traits specific to the person.

A therapist can treat the symptoms you have that you consider disabling without giving you a formal diagnosis. In my opinion, this is probably the way it should be. You don't need to be in a special category to get help.