Legal age to stop paying child support?

I have a friend that asked me to search this for her. She has two daughters ages 17 and 21. Her ex has always paid child support for both girls until the older daughter turn 18. She went to college and is now on the Dean's list. However since she turned 18 her father has stopped paying child support for her. Now the youngest daughter is turning 18 and on her way to college. My friend and her ex were never married and the father pays the child support through the state child support agency.