What age can a child legally decide where to live?

It is a complicated area of law. Generally the courts give increasing weight to the views of the child as the child’s maturity develops – so for each child the answer is different. Generally speaking for girls age 12 to 13 is when the court will start to give more weight and importance to the child’s wishes for boys more like 13 to 14. On your facts, the grandparents would have to be applying to court to seek primary care of their grand daughter asn the child’s views would likely become known to the court through a report from a counselor who can say what the child’s wishes are and on what basis those wishes have been made. If the step father has mistreated her (by being abusive) that is something you can report to the Ministry of Children – who can investigate to see if there is a protection issue.