Can I file a motion to modify my child support if one of my children turn 18 years old ?

Unless otherwise agreed, child support ends at 21. Your child turning 18 does not in and of itself qualify you to reduce child support. Facts to consider in determining whether you can seek a modification: is your child residing away from the home of custodial parent, is he or she attending college away and you are contributing to those expenses (e.g room and board)? Is your child employed full-time? This is not an exhaustive list. You should consult with an attorney to determine whether any qualifying events have occurred so that you can seek to modify support.

You don't file a "motion" to terminate support and New York's age out is 21, not 18. See a lawyer if you have relevant facts about how independently the child lives away from his custodial parent.

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Pursuant to the Child Support Standards Act, child support terminates when the child is 21-years old or is earlier emancipated. 18 is the age of majority, but not the age of emancipation. Does the 18-year old work, live with a significant other, moved out of the home of the custodial parent, in college?