How old does a child have to be in California to decide which parent they want to live with?

California Custody Decisions

If both parents are equally acceptable in the opinion of the court, the request of a teenager would be given serious consideration.

Check out Family Law Code 3042. (a) If a child is of sufficient age and capacity to reason so as to form an intelligent preference as to custody, the court shall consider and give due weight to the wishes of the child in making an order granting or modifying custody.

(b) In addition to the requirements of subdivision (b) of Section 765 of the Evidence Code, the court shall control the examination of the child witness so as to protect the best interests of the child. The court may preclude the calling of the child as a witness where the best interests of the child so dictate and may provide alternative means of obtaining information regarding the child's preferences.

The Family Court judge would decide if the child(ren) are mature enough to decide why they wish to live with the parent named. Age is less of a factor then the ability of the child to voice a valid opinion other than they simply "want to do it."