There is no set age in Illinois at which you can choose with whom you live. Your thoughts and preferances may (only may) be considered. If you wish to live with your dad, he will have to file a motion for transfer of residence (or custody, depending upon how the divorce decree reads) and must allege facts sufficient to be considered by the court if your mother does not agree. Your dad needs a lawyer to do this. Best wishes to you.
Your dad needs to retain an attorney and file in court to change where you live. Often the court will talk to the children about your desire to live with your dad. The court can also appoint an attorney call a Guardian Ad Litem to Represent you in this matter.
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I answered this very question on my blog a few weeks back.
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