at what age can a child declare which parent they want to live with. My mother wants my niece to say that she doesn't want to live with her mother. She thinks that my niece is allowed by law to make this decision at age 12. Is this true?

Under Arizona Statutes, courts look at several factors to determine which parent should have custody of the child. A.R.S. 25-403 provides that the court considers

1. The wishes of the child's parent or parents as to custody 
2. The wishes of the child as to the custodian 
3. The interaction and interrelationship of the child with the child's parent or parents, the child's siblings and any other person who may significantly affect the child's best interest. 
4. The child's adjustment to home, school and community. 
5. The mental and physical health of all individuals involved. 
6. Which parent is more likely to allow the child frequent and meaningful continuing contact with the other parent.
7. Whether one parent, both parents or neither parent has provided primary care of the child
8. The nature and extent of coercion or duress used by a parent in obtaining an agreement regarding custody.

Thus there are many factors that a court will consider. The court considers a child's wishes but no specific age is specified in this statute. See question #152 to find out more about rights of step parents, #22788 for more information on custody.