I started showing signs of BPD when I was about 16, but it got much worse when I was 20-22.
My best friend has BPD too, but I don't know too many details, all I can say is that he has a 3 year-old daughter, and while we can't say anything for sure, she already shows signs of possibly developing BPD later on. Her younger sister doesn't and when spending some time with both of them the difference is really obvious. Because she's only 3, all we do is try and teach her how to deal with strong emotions and hope for the best.