“If a child is already 18 years old, can a father be made to pay child support? If so how many years would the father be required to pay back pay?

(I live in the state of Tennessee and there has never been a child support order)”

I’m going to answer under California law, but it would not surprise me if the law is the same in Tennessee. Child support continues until a child is both 18 and graduated from high school or 19 in any case. Child support orders are only modifiable back to the date of filing. Where, as here, no case for support has been filed, the Court cannot go back before the date of filing to a date when it could have ordered support and then order ongoing or back support. So chances are good that this father cannot be ordered to pay “back pay” prior to the date of filing.

Goevrnment assistance actually paid could be collected even if the child had already turned 18, but that would be by a governmental unit. This would not be an action brought by either parent, but by a local child support collection agency.