How old do children have to be to decide which parent they want to live with?

Children don't get to decide. The judge does. The child's choice will be weighed by the judge, but inevitably the court will make the decision that is in the best interest of the child.

Also, the age is dependent on your location.

Thank you and I'm in the UK

First, please pay more attention to posting guidelines. There is a Read First sticky at the top of the Children forum (where I moved this from) that refers questions of a legal nature to this forum.

Second, I do understand the desire for anonymity, but since this site is international, it really is important to give some indication of a general area.

I don't believe the laws in the UK are different. But I know the procedure isn't. If the child wants to change custodial parents, then the non custodial parent needs to go to court to modify the custody order. The court will hear the arguments, including talking to the child. The court will then make a decision based on what it feels are the best interests of the child.