In order for your mother to get support, she's going to have to take you to court. Plain and simple. She's not his parent, and you were never ordered to pay support to her. She can't just "demand" from you and get support for someone who isn't even her child. Tell her if she wants child support, to take you to court, and let her spend her money. Your son is 18, he moved out of your home. You say she's having papers served on you to get "legal" custody of your son. Thats different from "physical" custody. And at either rate, your son is 18. He's an adult in the eyes of the law. When these papers are served on you, you need to file an answer to them. What you need to put in that answer is that your child is 18, and is emancipated. This is from the Missouri statutes...
"(3) The obligation shall be deemed terminated without further judicial or administrative process, when the parent paying child support files a sworn statement or affidavit with the court which entered the order establishing the child support obligation, or the division of child support enforcement, stating that the child is emancipated and reciting the factual basis for such statement; which statement or affidavit is served by the court or division on the child support obligee; and which is either acknowledged and affirmed by the child support obligee in writing, or which is not responded to in writing within thirty days of receipt by the child support obligee"
Yes, this is for PARENTS who are paying CS. But you need to file the statement saying that your child is emancipated just the same, and state the reasons for such. I.E. He voluntarily moved from your home, has his own job and is self supporting, is 18. Make 3 copies of your statement. Send one to the court, the other by certified mail, return receipt requested to your mother, and keep one for yourself. What IAAL stated is correct. Your mother has no legal rights to CS. But, if she insists on filing these papers, then you need to be prepared to answer them, and state that your "child" is no longer a child.