In Oklahoma a child normally must be 18 years of age to decide where to live independently. On the other hand, a younger child can be ruled emancipated and allowed to make such decisions. And if you are talking about which parent the child chooses to live with, judges have great latitude in deciding whether the child's wishes will be considered. There simply aren't enough facts provided to adequately answer your question.
I think you may be referring to the following Oklahoma statute, which raises a presumption at age 12:
Okla. Stat. tit. 12, § 113:
"A. In any action or proceeding in which a court must determine custody or limits to or periods of visitation, the child may express a preference as to which of the parents the child wishes to have custody or limits to or periods of visitation.
B. The court shall first determine whether the best interest of the child will be served by allowing the child to express a preference as to which parent should have custody or limits to or periods of visitation with either parent. If the court so finds, then the child may express such preference or give other testimony.
C. There shall be a rebuttable presumption that a child who is twelve (12) years of age or older is of a sufficient age to form an intelligent preference.
D. If the child is of a sufficient age to form an intelligent preference, the court shall consider the expression of preference or other testimony of the child in determining custody or limits to or periods of visitation. Interviewing the child does not diminish the discretion of the court in determining the best interest of the child. The court shall not be bound by the child's choice or wishes and shall take all factors into consideration in awarding custody or limits of or period of visitation.
E. If the child is allowed to express a preference or give testimony, the court may conduct a private interview with the child in chambers without the parents, attorneys or other parties present. However, if the court has appointed a guardian ad litem for the child, the guardian ad litem shall be present with the child in chambers. The parents, attorneys or other parties may provide the court with questions or topics for the court to consider in its interview of the child; however, the court shall not be bound to ask any question presented or explore any topic requested by a parent, attorney or other party.
F. At the request of either party, a record shall be made of any child interview conducted in chambers. If the proceeding is transcribed, the parties shall be entitled to access to the transcript only if a parent or the parents appeal the custody or visitation determination. "