Do I stop paying child support once my daughter turns 21 years old?
Child support continues until the child is 21, or when the child is emancipated by the Court. The Court can emancipate a child prior to the 21st birthday if he marries, joins the military, becomes self-supporting, or is over 18 and has not been enrolled in school for more than 4 months. The Court could have ordered you to pay support and your fair share of the child's college expense. Such an Order can continue even after the child is 21. You need to read the Order for specifics on this issue.
You might consider notifying your ex-wife and daughter of your understanding as to when your responsibilities pursuant to the Order ends.