In Ontario, the second they are adult. Until then, if there is a custodial issue or if the child wants to try to change custody, they can use what's called a "Child Lawyer". Here in Ontario, these people are in effect working for the child, neither parent. They can appear on behalf of the child during custodial matters in court. It's not to say that they'll win and the child get's their own way, but they are there to explain to the presiding judge, where the child wants to go. In any event, the Judge will always have the best interests of the child in mind. If the Judge feels that what the child wants as far as custody is concerned, is not what's best, he will deny the child's application. If you have a current, formal custody agreement and it was worked out by a Judge or a mediator, chances are, it won't get changed by the child alone. Many Judges feel that age 12 is where they'll start putting more weight on what the child wants. So if there isn't any custodial agreement now, your eldest will have some say on where he/she lives. The Judge may agree, he may not but he will listen to what the child has to say and probably WILL ask the child.