I can speak from my own experience. I've just been diagnosed with BPD. I'm almost 40 years old. I had always wondered why I do the things I do, react the way i do. It took a very bad interpersonal relationship with someone else where they called me out on my behaviors to start to look into things. When I started reading, I came across borderline and as I read more and more, it was like I was putting puzzle pieces of myself together. From there I went for official diagnosis and no doubt in my mind, the results came back just as expected.