Notwithstanding any other provision of law, no person convicted of murder, rape or any other serious felony, as defined in Section 1192.7 of the Penal Code, committed when he or she was 18 years of age or older shall be committed to Youth Authority.

The provisions of this section shall not be amended by the Legislature except by statute passed in each house by rollcall vote entered in the journal, two-thirds of the membership concurring, or by a statute that becomes effective only when approved by the electors.