I have histrionic personality disorder that, as many of you probably know, is from the same cluster of personality disorders as BPD, so we share many similar traits, i identify with many of the content posted here, etc.. just to clarify.  
So, I've read that personality disorders usually appear during/after adolescence because they say it's when you actually have already "formed a personality". But i remember having symptoms as a child. I remember the chronic feeling of emptiness, the terrible anxiety, the constant need for attention and validation... As a matter of fact, I noticed these traits as a separate thing and used to see it as just "being immature" i thought I'd grow out of existencial anxiety, of constant need of validation and attention, of wanting to flirt even with people I don't intend to have anything with.  
I would play these scenes in my head where everyone would realize how cool and pretty and intelligent I am (i still do, but in a much shorter scale than it used to be). And i thought once I was old enough to live freely (controlling parents etc) I would be able to not look out to people so much and be less insecure.  
But this "coming of age" i expected never happened. And it's because it was never being immature, i had terrible anxiety and onset HPD but didn't know. How could I know? Even after all these years I wasn't able to be fully honest with my therapists, i didn't want them to see how ridiculous I can be. I didn't want to talk and then I didn't know how to talk.  
P.s.: if you did start presenting symptoms only later in life, I'd also like to hear what it was like for you!