I believe they can occur at all ages, but usually they don’t like diagnosing until about 18. At 18 you have a long enough life graph that they can look at, and by then your personality is usually fully developed. I know I had a hard time convincing my doctors that stuff I did in high school was related to BPD because it’s normal for teenage girls, but it was so out of my character that once they got to know me they agreed. (I’m this timid docile innocent person that would have sex to get revenge on a friend… which I would feel horribly guilty about)
Anyways! It can start younger then 18, but you usually wouldn’t see a 30 year old say “I just started having BPD symtoms” because it’s something you grow up with. Although it took my mom until like, 33-ish to get diagnosed. It’s a very tricky mental illness.
Best Regards,
Chelsea.

Age of Onset :
The symptoms of borderline personality disorder usually first occur in the teenage years and early twenties. However, onset may occur in some adults after the age of thirty, and behavioral precursors are evident in some children. ” (http://www.bpddemystified.com/index.asp?id=2)
Personally it started when I was a child.

- Diagnosed with social anxiety and depression, add : “You could tell your social worker that you came across a community of people suffering from BPD and that you relate to their experiences and feelings. Then, be prepared to explain why you think you have it. Prepare yourself, take the list of BPD symptoms and explain which of those you think you have and how they affect you.

Love,
Julie