This is a trick question. There is no age at which a child can make these decisions, until he/she turns 18 and the court no longer has jurisdiction over them.
There are, however, a few methods by which a child can have input in the process. 1. An attorney can be appointed for the child. If the child is old enough, the attorney can advocate for the child’s wishes; 2. If there is a custody evaluation, the child will be interviewed and the evaluator may take the child’s views into account; 3. The child can testify in court. I never have children testify and am opposed to it. Judges do not want children to be involved in the process and it is generally thought that the less a child can be involved in the process, the better. That being said, an attorney can help a parent to advocate for their child’s wishes.