Generally, child support ends when a child enters active duty military service. Once that begins you should file an Affidavit for the Termination of Child Support (http://www.courts.mo.gov/file.jsp?id=639).
If you have additional questions, contact an attorney that has family law experience.
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C. Curran Coulter II, J.D.
Attorney and Counselor at Law
THE COULTER LAW FIRM, LLC
8000 Maryland Ave., Suite 1060
St. Louis, Missouri 63105
Office: (314) 721-1116
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Legal disclaimer: The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of Missouri and the United States District Court for the Eastern District of Missouri. Responses are based solely on Missouri law unless stated otherwise.
If the child is already 18 and not in school you can file the affidavit now. If he is still in school you have to wait for him to finish or enlist.