Can I sue for back child support even if the children are in college?

I am owed over $15K in back child support. I have been struggling with my kids ever since they were born. They are in their early 20s and is in college. When the youngest child turned 18, I got a letter stating there was money owed in back child support and that was in 4 years ago. The father claims he is getting disability and I don't know if that is true or not. I still take care of my kids and I need help with their college expenses because the schools they are planning to attend is expensive and pell grants and loans aren't enough and neither of us are credit worthy of getting a private loan.