Legal Question in Consumer Law in Pennsylvania

Its YOUR account. You are responsible for all charges incurred on YOUR account. The minor could only avoid contracts entered into by the minor on the minor's account.

More details are needed here but it sounds as if you gave the minor permission to use your account or add a line and the minor either downloaded expensive junk or made some other changes which you only found out about now. If that is the case, you are responsible because you allowed the minor to have access to your account. If the minor was a total stranger and engaged in some kind of fraud to make charges to your account, that is different. The minor could be prosecuted as a juvenile and Sprint would need some kind of fraud affidavit and maybe a police report if that happened.