Need more facts on the situation. does this person live with either parent. What are the parents doing to have the child return. do the parents care? A child can be emancipated if the child can support himself, and do the other things an adult needs to do to not become a ward of the state or engage in criminal conduct. Once the child is 18, he is on his own. Unless he stays in school and he graduates later, then he might want the benefit of staying a parent's home to finish school so at least he has a high school diploma. If life is absolutely horrible, the child should contact child protective services for assistance until he turns 18 and to make sure he at least graduates high school.
When a child turns 18 they reach the age of majority. Until that time there is a legal presumption that the parents are acting in the minor's best interest and can seek court assistance in forcing the minor to do what they determine is best. However, looking at this matter rationally, it is unlikely that the Police would arrest the minor (however that is a risk however unlikely).
It is more likely however that the parents would have to begin a legal battle in order to have the child returned to their care.
If the parents bring a legal action or the Police assist in returning the minor, DCFS could be contacted or the Minor could contract her own attorney for protection or to seek emancipation. However, Utah County Courts (and I think all courts in general) are reluctant to grant an emancipation and again, the minor would 18 before the matter would be dealt with.