Protection of employees is weaker than anywhere else in the US. The Northern Mariana Islands have been accused by the US Federal Government and various other organizations for enslaving foreign workers (particularly from China and the Philippines) and immigrants in sweatshops and prostitution.
It can be as low as 18 or as high as 21 depending on where the child lives. In Delaware, the age of majority is 18. However, almost all states allow the child, a parent or a guardian ad litem to write to the judge and/or petition the court for modification of the original custody agreement.
The only state that allows a child to choose is Georgia at 14 and this is only assuming that both parents are fit. Most states do allow at around 12-14 to speak to the judge and express wishes but as stated above their wishes are only considered and decision is not solely based on that.