Unfortunately, the law regarding a child's 'right'* to choose which parent to live with is murky, and varies considerably by State and jurisdiction. Although not a standard by any means, many States have begun to give 'consideration' to a child's declaration of custodial preference when the child reaches the age of twelve or thirteen. The judge is normally given almost unlimited latitude in whether or not he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" when a child can say who they want to live with. In most cases the circumstances of the situation will matter as much or more than the child's age.
It is true, however, that the older the child is the more his or her preferences tend to influence the judge. For example, forcing a child of fifteen to remain in a home where he or she does not wish to be could quickly lead to more serious family problems. This 'harmony' factor may carry more weight in the judge's estimation than the child's preference itself.
Some States allow an "Affidavit of Preference" to be signed by a child to specify a custodial parent or Conservator. Again, the judge is given almost unlimited latitude in how much importance he attaches to the affadavit. Some may disapprove of it, feeling that the child has been unfairly put in a position of having to choose (and perhaps rightfully so).
Generally speaking, there are a few criteria that judges typically consider to one degree or another when adjudicating a case where the child or children have strong preferences:
What is the reason or reasons the child wants to change residences? Is the reason a valid one? Is it a significant factor or issue in the child's life? Some issues have more importance than others, such as schooling or a need for specialized medical care.
What is the level of stability and reliability of the parent the child wants to live with? If that parent is unstable or unreliable the judge will not likely approve the change. The parent the child wants to live with must be as capable and as able to care for the child as the one the child currently lives with.
What is the level of the child's social maturity and emotional and intellectual development? Judges will be more receptive to a child who appears able to understand and deal with the changes involved in a modification of the custodial status quo.
How does each parent support the child's decision? Is the move being made against the wishes of either of the parents? Has the child been pressured, bribed, or manipulated into the move? If a judge senses that there has been any "behind the scenes" interference, it would be extremely unlikely for the move to be allowed.
With the move truly serve the child's best interests in the long-term? If a clear and significant benefit from the change in residence cannot be demonstrated, a judge will be unlikely to give approval. The benefit(s) must be obvious and long-term.
Can the child clearly articulate his or her reasons for desiring to move? The more logically and lucidly a child can explain why they want to change residences, the more likely it is that judge will pay attention to their preference. If the child appears uncertain, confused, or insincere, the judge will ignore or greatly discount what the child says.
In some cases the judge may decide to talk with the child privately "in chambers". Attorneys and parents are normally excluded so that the child can speak freely without fear of displeasing one parent or the other. It is worth noting that what a child tells or relates to a judge in chambers may not necessarily be kept confidential. No one, not even the judge, can guarantee that what a child says will remain confidential; indeed, the child's statement(s) may play an important part in the judge's decision and as such would normally become a matter of record.
Unless circumstances leave no alternative, children should never be asked or required to testify. Even having the child speak privately with the judge should be avoided. The stress that testifying places on a child is immense and unfair- even a 'private' talk with the judge in his chambers is testifying in some way, and the child knows it. No child wants to be placed in the position of being asked to choose one parent over the other. Only when the child has a genuine and voluntarily desire to speak with the judge should it be considered.
* Technically speaking, the child has no legal 'right to choose'. Minors are, by definition, "legal incompetents", which means they are not recognized by the court as being able make legally binding decisions, including decisions regarding their custody.