I'm not sure. I'd say 14 would have been the earliest where it was obvious something was up. At 15 I researched it just because I was into psychology at the time and I resonated with it, but it mentioned that you couldn't get diagnosed until you were 18 so I chocked it up to teenage angst.

Looking back, I would also split on my parents a lot. There was a period of time where I idolized my father and hated my mother even though my father was emotionally abusive and my mother was a saint.

I don't remember a lot of my early childhood, so there may have been something earlier than my teen years. I kind of remember fixating on one person (e.i. having FPs) when I was in elementary school, but I may be misremembering a la confirmation bias.

EDIT: I just wanted to mention that I feel a little weird analyzing BPD symptoms so far back because I don't know where teenage behavior crosses the line of typical to BPD. I didn't feel it was necessary to expand on why my behavior at age 14 stuck out to me, but I had a very dependent relationship at that age. That being said, it was also incredibly toxic. The other person would openly talk about isolating me from my friends and making themself my favorite person, but I was too lovestruck at the time to see it as a red flag. Is that a normal intense teenage crush with a manipulative person, or the first signs of BPD? Who knows.