There is no magic age in Illinois. The judge can take into account the opinions of the child and place whatever weight they wish on their opinion. In my experience a child in high school's oppinion usually matters more than other ages. Of course this is tempered by the best interests of the child standard. A child at any age may not know what is there best interest. For example a teen may want to live with the less strict parent for obvious reasons when the more strict parent would be better for their development.
As to you question regarding a parent requestion a court appointed guardian i am unsure to what you are referring. Sometime a court may appoint a guardian ad litem for minors to represent them in proceedings when the court fears there interests are not properly represented by the parents. This is something a parent can request af part of a determination of the best interest of the child, and the procedures are provided for statutorily.
As far as options are concerned you can file a petition for modification of the custody award. If two years have not passed since the original order was entered you will be required to demonstrate a substantial change in circumstances that warrant the review by the court. This prevents parents from bringing issues to court every other month when a child is angry at one parent.
The judge will decide. There is no magic age when children get to decide custody. The best interest of the child is the standard.
The law regarding a child's 'right'* to choose is a matter for each State and jurisdiction. The judge in most States, not the child, makes the decision based on the best interest of the child. 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, sometimes fourteen. There are even cases when children of age 9 are allowed to testify.
The judge is normally given almost unlimited latitude in whether or not she or he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" but the younger the child the less likely for a judge to give the stated preference much weight.
Good luck to you.
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