When a child reaches age 18 they are considered to be legally an adult in most states.
So that means that you won't have to pay child support to your child or child's mother when the child reaches age 18.
Also if the child gets emancipated then usually you do not have to pay child support anymore then either because the child is considered legally able to take care of themselves.
A child dropping out of school is also grounds to terminate child support payments because when a child drops out of school then they are considered able to take care of themselves.
Child support has to be paid until the child reaches the age of majority which means that the child is no longer considered legally a child.
18 is the age of majority in most states but check with the court first to be sure you can stop paying child support payments when the child reaches age 18.
But usually when the child turns 18 or older then child support payments are no longer required.