How old does a teenager have to be in order to choose which parent she wants to live full time, in a joint custody case?.

Technically speaking, a child under the age of 18 never gets to "choose" which parent with whom to reside. The "wishes of the child" are currently one factor among many that the court must consider in making a determination about the "best interests of the child." Certainly, an older child's wishes will carry more weight than a younger child; however, it is never the child's decision.