Law should provide the means necessary for the victims (direct and indirect) to be repaid for the damage suffered. Therefore, the age of those who committed the crime(s) should not matter at all, for the damage done to the victim and society remains of equal amount and importance.
It´s a fact the law was designed to protect people under 18 due to their mental maturity but it is possible to assume the maturity necessary to commit the crime is equivalent to maturity necessary to understand it´s consequences and to accept responsibility for one´s actions.