How old must a child be to legally express which parent they would prefer to live with?

Where custody, care & control decisions are concerned, in the absence of agreement between parties, the Court has to make a decision with the welfare of the children in mind, based on the evidence made available in the affidavits (i.e. parties' accounts, counselling reports etc.) There is no fixed age at law or any fixed procedure for the Judge to interview children about their wishes, but I would say that the Court is more likely to take into account the opinions of children in their late teens.