Under Maryland law, you can't stop paying support until your child at least reaches the age of 18. If he turns 18 after the date of his high school graduation, your support obligation ends. If he turns 18 during his senior year, you're still obligated to pay until he turns 19 or graduates, whichever occurs first. Support also terminates if your child quits school after his eighteenth birthday, but before he turns 19.
The Maryland legislature attempted to pass legislation in 2011 that would have made parents responsible for paying child support until their child turned 21 if he attended college or until college graduation, whichever occurred first. Known as House Bill 986, the effort didn't pass. A similar effort, House Bill 1272, would have obligated parents to contribute to education costs in addition to paying support during the college years. This initiative also failed. As of 2012, Maryland courts can only order child support past the age of emancipation, or contribution to college costs, if parents agreed to such terms in a marital settlement agreement. A judge can enforce the agreement, but the Maryland state code does not mandate child support past the age of 19 or high school graduation, whichever occurs first.
Maryland law is vague regarding how to terminate your support obligation after your child emancipates. The state requires that all obligor parents pay through state services, so when your child becomes emancipated, you can notify the state that you no longer owe child support. If your ex-spouse doesn't dispute it, you may not have to take any further action, but speak with a lawyer to be sure. If your ex does object, you might have to file a motion with the court to ask a judge to determine if your child is emancipated. If you're supporting more than one child, this will almost certainly necessitate a motion to modify support because the emancipation of one child is not likely to terminate your obligation entirely. It would probably reduce your child support payment, but you'd have to ask the court to recalculate your obligation based on how many remaining children need your support.
If your child is disabled, either mentally or physically, so he cannot possibly support himself, your support obligation can extend past age 19 or high school graduation. It may be indefinite, for as long as he needs you. Conversely, your support obligation could end before he turns 19 or graduates, if other milestone events occur. For example, if he joins the military before graduating from high school, you don't usually have any further legal obligation to support him. If he marries or leaves your ex's household to live on his own, with your ex's consent, you can file a motion and present a case to the court that he's emancipated.