Typically, a non-custodial parent is only going to have to pay child support until a child turns 18 and/or graduates high school. If a child turns 18 before graduating, the parent will likely have to continue paying child support until they graduate.
As well, if the child is disabled in some way that prevents them from taking care of themselves, the payments may be prolonged by the courts indefinitely. Other factors that may come into play when determining child support orders after the child turns 18 may include expenses as a result of their college education.