In many states there is not a firm age at which children get to decide which parent they live with. Instead, the judge in certain circumstances may consider the child's preference as one factor among many. The only way to know for sure is to speak with a lawyer in your area.
In North Carolina, a child's wishes is just that - their wishes. A judge will consider their wishes but the child does not decide which parent to live with. Were the parents to allow the child to make the decision, they would be allowing the child to take the role of the parent. It is the parent's job -- and the Court's in a custody case -- to decide what is best for the child.
The court will consider the child's wishes as merely one factor. Most children age 9 and older are able to express a preference. Some counties will allow the appointment of a guardian ad litem to determine the child's wishes.
Once a child has a driver's license, it is tough - however - not to allow the child's wishes because if the parent is not aiding and abetting the child in violating a court order and the child violates the order, the judge has no real choice but to punish the child. So at 16+, unless circumstances strongly dictate against it, the child's wishes actually do matter much more than at an earlier age. Despite this, I have won at trial to have a child of this age live with the more responsible parent against the child's wishes.... The Court MUST Consider the best interests of the child unless one parent is unfit or has acted inconsistent with their constitutionally protected status as a parent.
Check with a lawyer in your county for details about how judges in that county see this issue.