It isn't a matter of "what age" -- regardless of a child's age there is no "choice" to simply be made by the child -- there are ages (vary by state) at which a child's opinion may be considered by the court in making the decision of residence but it is not a matter of "well, I turned 12 so bye".
If the child reaches the age where their opinion is considered and desires to move then there would have to be a petition for change of custody (brought by the NCP most likely) and the child's desire would be listed as the reason. You would oppose the move and that would result in a judge hearing both sides of the issue and making a decision of what he/she feels is in the best interest of the child in question.
Of course, all of this is null and void if there is anything in your order that addresses this subject......
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