Retroactive child support in Illinois can be sought retroactive to date of birth. Visitation and child support are unrelated; however, you are not immediately subject to retroactive support. A number of factors will determine that at a hearing.
If she has filed a case here in Illinois, you'll want to retain an attorney to assist you in establishing your parental rights, obtaining a visitation order, possibly fighting for custody, and ensuring that the court fairly calculates any child support obligation.
James Rowe is an attorney with The Law Firm of Rowe & Associates in Illinois. He can be reached at (312) 345-1357.
I respectfully disagree with my above colleague on this issue to the extent of "when" support can be backdated.
Although it is "theoretically" possible for her to seek backdated support, it's almost impossible to get. 99.9999% of the time it's backdated to the date the petition was filed.
Either way, talk to an attorney in your area. Contact the local bar for a list that fit your needs, and best of luck!