Adopting an 18 year old

My wife and I are considering a form of adoption that's so rare I can't find a forum in which to discuss it. So ... I'm going to try soliciting some feedback here. My wife and I adopted our first daughter in 2001 and our second daughter and two sons in 2006. None of them were infants. The youngest was 2.5 at the time of adoption. The oldest was 11. After adopting four kids, we felt content and did not plan to adopt again. Then, in 2010, we met this girl. She was a friend of a friend of our daughters. She was 13. She had recently moved in with a sort of step grandmother. Both of her parents were/are alive and live within a few miles of us. I'm sure both of her parents love her in some way, but they do nothing to show it, and unfortunately can't seem to take care of themselves, let alone her. It's a shame, because she's a remarkable young lady. I'll spare the details of how we got to know and bond with this 13 year old girl, but we did, and in 2012, within a week of having turned 16, she moved in with us. My wife and I became her permanent legal guardians. We wanted to be more, but she was not a part of the foster system, her parents' rights had not been terminated, and her parents would not consent to her adoption by us. Fast-forward two+ years: The girl of which I speak is so a part of our family. We have been through quite a lot together. She recently turned 18. She's indicated at times that she wants to be adopted. She's also indicated at times that her parents will never consent so it's probably not worth the effort. She's so a part of our family that my wife and I think we're ready to make the effort, and we think she is too. We could do an adult adoption here in Colorado, and as part of the adult adoption we could change her name to match ours (which she already uses except on formal papers). Her parents don't need to consent. It would be really, really easy and quick to do. But from my perspective, I want more. I think she may want more too. Yes, my wife and I need to talk to her about it, but before we do, I wanted to get some feedback via this forum. The other kind of adoption we could still do is a child adoption (yes, it's possible until the adoptee turns 21 in Colorado). A child adoption may expose all of us (and mostly the young lady we want to adopt) to some trauma, as we would have to terminate her parents' rights. If her parents contest the proceeding, that will be traumatic - especially to her. If her parents don't contest the proceeding, that too will be traumatic (i.e., more Why?s that she'll never have the answer to). The benefit of doing a child adoption is that she'll not only be our daughter, but her children (if she has any, which she probably will) will be our grandkids. Why does that matter? Well, hopefully it doesn't. But, should she someday have children and no husband, and should something terrible happen to her, we would only have standing to step in and care for those kids (to keep them out of foster care) if we are their grandparents. An adult adoption of our daughter does not make us the grandparents of her kids. An adult adoption, as I understand it, is pretty much just a court-endorsed contractual relationship between us and our daughter; whereas a child adoption changes her birth certificate and actually intertwines our family trees. Plus, I think our un-adopted daughter suffers from a bit of an identity crisis, in which she's not really sure how to describe her relationship with us. Making her 100% a part of our family seems like it might help her - both now and throughout her life. One more datapoint - she's a Type A, and although all of this stuff is traumatic, I think she would get through it better than most. I've read articles about foster kids who age out of the system without ever having a family, wishing they had a family. I've also read articles about adoptees who wish they had never been adopted. And I've read all kinds of articles in between. However, it's next to impossible to find a discussion about adopting someone between the ages of 18-21, and whether it's a positive thing or a purely negative/traumatic thing. Thoughts? If we discuss this with our daughter, do you think she'll say, "Yes, they really love me and are committed to me forever," or will she say, "Great. One more thing to worry about it." And yes, I realize she may or may not come out and say either to us, but what do you think she'll be saying inside if we tell her we want to file to adopt her as our child? I would appreciate your thoughts. Thanks.