I guess we as a society believe that a minor who murders still deserves some protection, inasmuch as they are still children and don't reason like adults.
We don't allow them to enter contracts based on the same reasoning; we assume they don't have the reasoning capacity necessary to make rational decisions. Similarly they can't vote. They are prevented from taking high risk jobs, can't make their own medical decisions, and so forth.
Now if they are "charged as an adult" I think the adult rules come into play. I guess the D.A. makes that determination.