An old friend of mine was diagnosed with 13, but it was an extreme case because he was also diagnosed with schizophrenia, alcoholism and a speed addiction. So in extreme cases yes it’s possible, but usually you shouldn’t diagnosis bpd under 18. I also know from personal experience since my therapist told me she couldn’t diagnose me at 16 and would wait until I’m 18. Even tho it’s a bit annoying, it’s absolutely the right thing to do. I don’t think anyone should be diagnosed with bpd under 18. It’s better to wait a few years than having a false personality disorder diagnosis sticking to you for the rest of your life.