19 Year Old Adopted??

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I was talking to a mate on Facebook who I knew was going to be having someone new live with him and his partner but what I had not been told was that him and his partner were planning on adopting this girl even though she was 19 and they were 25 and 21 respectively, I was really confused why they would do this and his answers were not reasons why they should, My reasoning was that she should just come and live with them but he didn't think that would just work. He then had a go at me for questioning this when I was talking to him on Facebook and his partner was really mad at him and told me that she was just going to move in but it wasn't at all convincing. what are your thoughts??