From what I've read, BPD sometimes develops (or surfaces) after the age of 18, sometimes from a "traumatic" event which I believe is what happened to my daughter. She was PERFECTLY fine all her years with us, through school, great at home, etc. THEN she went to college and so it began... .
I did not think it was "traumatic" as she was only less than maybe 110 miles from home and I saw her very often. But something being in the dorms, around other girls whom she may have thought were "better" than her in some way, I don't know... .it just broke loose. She got a medical excuse to get out of the dorms early and moved in with 4 girls, all of whom had depression or some other illness, and she seemed to take on the "traits" of the girls. Soon she was skipping classes, then went on to the cutting and suicidal attempts, many hospitalizations, in and out and in and out of schools and clinics, wracking up thousands in bills. Her dad and I were beside ourselves in the beginning, spending $$$$ to pay her rent and bills and loans and everything we could do because she was "ill." Even after diagnosis, it just couldn't register with us how a totally happy, outgoing girl left us at 18 and became this person we didn't recognize.
I STILL (she's almost 25) don't know the "official, DSM textbook diagnosis" although I have spent hours and hours and hours reading, researching, etc. Most of all, she just keeps us hopping STILL like puppets--and I know she loves it, and I admit, I don't know how to totally STOP. Her dad has, he is very detached although he worries for me so IF he caves for money or whatnot, it's for MY sake, not hers, and I hate putting him in that position.
Very much at the top of the list is her not ever, ever, EVER being able to say "I'm sorry" or admit she's at fault with ANYTHING. It's always someone else, or a circumstance, or the weather, or she's sick and couldn't do it... . Also empathy is an issue, although she seems to have it for other people (unless she's faking that, too?). I once told her I am so wrapped up in her life that my doctor told me (after my THIRD blood pressure medication added) that I am just asking for a stroke or heart attack. She looked right at me and said "I DON'T CARE, aren't we talking about ME now?" I couldn't believe it.
She knows how to get what she wants/needs from whoever is willing to give it to her at the time. If **gasp** we put a boundary up, she moves on (or in) with someone after the story we "kicked her out and she was homeless." NEVER happened (although hubby did threaten and print out a list of homeless shelters). All we wanted was for her to get a JOB (at nearly 25, I think that's a reasonable request, no?

But dear Lord, you'd of thought we asked her to cut off all her limbs or something. Her car (that we got her in high school, not meant to be her forever car) is falling apart, but she is "not going to stoop to taking PUBLIC transportation." Excuse me... .I did in my 20's. All the time. And did this awful thing called WALKING places.
Told me not too long ago she was "so ill with strep" and was on antibiotics, etc. and could I do this or that. I felt bad, of course. I called the clinic (I made her sign authorizations) and found out yes, she went to be seen for a sore throat, but everything came back Negative and NO Medications were given. So no strep, no antibiotics. They "cry wolf" a lot to get reactions from people and this is where it gets dangerous because they have to keep "upping the ante" and she could inadvertently do something to really harm herself without meaning to.
Not grateful for things, "expects" things because "I didn't ASK to be born!" No "thank you's" or very seldom, and never to her dad, just to me, even if HE gave her the gift. But I'll inquire if she received so and so, and she'll go, "Oh, yeah, I did. I was busy and forgot, but thanks." This was a girl we raised to be polite and thankful for gifts received, and notes of appreciation always needed to go out, that's just how it was/is.
Sorry for the rant; had to vent because we went to an event with her yesterday and as usual, it turned out badly, and with hubby only getting one weekend off a month, we really could have done other things. Long story short, not being able to admit a fault
ever is a concern, and I'd be watchful for other things: lack of empathy, manipulation tactics, EXTREME intolerance of even MINOR criticism, major changes of any sort, etc. Best wishes to you.

These boards are great, keep coming back.