The thief 18 years old was arrested and demanded compensation. Can he sue his family for compensation for his inability to pay?

First of all, whether the thief has money or not, the object of your civil lawsuit can only be the thief himself. The thief has reached the age of 18, and his parents are not obliged to pay compensation for him.

Secondly, it stands to reason that before the criminal trial, you should go to the thief himself to negotiate compensation, and then decide whether to sign a letter of understanding according to the specific situation, or you can give the procuratorate a sentencing plot. Now that the case has been decided, if the thief insists on not returning it, and you don't have any evidence of the thief's personal property, even if the civil lawsuit finally wins, it will be difficult to get the money.