See link provided below for a listing of state-by-state adult adoption laws.
Yes they do.
The person is now an adult and therefore an adoption could not be "reversed' or expunged. The adult can change his or her name back to their birth name or to any other they might choose.
The laws on this vary from state to state; inquire about adult adoption with your state's equivalent to the "Department of Children and Families".
Talk to an adult you trust and consider adoption.
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The teen and the child will be taken by the adoption service.
Generally, yes. However, each state has its own laws regarding the adoption of an adult. You need to follow the law in your jurisdiction.
Yes, it is possible for a 25-year-old adult to be adopted by a US citizen who is her aunt. Adoption laws vary by state, so it would be necessary to consult with an attorney who specializes in adoption to understand the specific requirements and process in the relevant state. Additionally, the adoption process for an adult is typically different from that of a minor.
Cannot without official adoption.
You would have to file a Petition to Terminate Parental Rights and get a judgment.
No, Justin Bieber turned 18 back in 2012. He is now legally an adult. He can no longer be put up for adoption, only disowned.