Is it legal?

Hi and thanks for writing. Once someone has turned the age of 18, they are completely legally on their own. They are allowed to sleep wherever and whenever they want. It is legally very liberating.  However, just turning 18 is not necessarily evidence that she is legally entitled to be on her own. It is possible that, despite having turned 18, she has been deemed incompetent or unable to take care of herself and thus has a guardian (or parent) appointed to her supervision. These are rare situations, but do occur.  Assuming this is not the case, she is legally allowed to sleep at your house. Of course, her parents are also legally allowed to get upset, threaten to kick her out, etc... Although attending high school at 18 may make her a dubious choice for a mate, it has no bearing on her legal capacity as an adult as far as I know.  Best of luck, Evan H. Nass, Esq.