Child Support for a Child Living on Her Own

My question involves child support in the State of: Mass

A bit of a convoluted question here and please forgive me if some of it doesn't make sense. None of it makes sense to me!

Child support is due until the minor is out of school. That I understand. The minor in question is 20 as of last week and graduated HS in 2012. The question is this. The minor moved out of her mothers house before she turned 19 and has been living on her own collecting SS and disability due to a spine injury. Her mother took her father to court for back pay from 2011-2012. The amount is $4200.00. If the minor has been on her own, collecting, how is it that he owes the mother this money still and the court agreed that he owes her this money.

Did that make sense?

If the money is owed under an existing, valid child support order, then the money is owed.