What state and or country would this be in?

Edit... Saw it was Illinois... my bad.

Usually one cannot file for child support for periods previous to the date of filing.

There is a variety of information for Illinois here (http://www.childsupportillinois.com/), but I don't see a specific answer to that particular question.

Is there an existing order or are you trying to create a new one after the child has reached the age of majority ?