I believe you are obligated to pay that support (as ordered). There is normally no "automatic change at age 19" either... you normally have to move the court to terminate child support or it continues until you do. 19 is really only the first available time that you can stop it. In your case, 21 would be the first available time to stop it (unless your separation agreement says something to the contrary), but since I don't have all of the facts, please consult an attorney.
The Child Support statute provides that for child support orders entered prior to July 1, 1997, unless a court finds that a child is otherwise emancipated, emancipation occurs and child support terminates without either party filing a motion when the last or only child attains nineteen years of age unless certain conditions exist. I would file a motion to be on the safe side, but you should have no obligation after age 19 years.