When the offender is a minor under eighteen years and his case is one coming under the provisions of the paragraph next to the last of article 80 of this Code, the following rules shall be observed:
1. Upon a person under fifteen but over nine years of age, who is not exempted from liability by reason of the court having declared that he acted with discernment, a discretionary penalty shall be imposed, but always lower by two degrees at least than that prescribed by law for the crime which he committed.
2. Upon a person over fifteen and under eighteen years of age the penalty next lower than that prescribed by law shall be imposed, but always in the proper period.