The age of majority in most states is 18, Alabama and Nebraska 19, Mississippi and Pennsylvania 21. Many 17 years old and younger leave the family home without parental permission before they are of age. Unless the parent(s) or legal guardian file a runaway or absentee minor report; or the minor becomes involved in a situation which comes to the attention of authorities very little is done. If the underage individual finds themselves in such a situation, the police can take them into custody, and resulting consequences (if any) will be determined in juvenile court according to the state laws.
A 17-year old can be taken into custody, (by force if necessary) and returned to the parents, legal guardian or a juvenile facility, when a court order is in place. If this were not true states would have no reason to establish age of majority laws that could not be enforced.