“The requirement to impose a sentence of HMP detention is based not on the age of the offender when sentenced but on the age of the offender when the murder was committed, and it reflects the humane principle that an offender deemed by statute to be not fully mature when committing his crime should not be punished as if he were. As he grows into maturity a more reliable judgment may be made, perhaps of what punishment he deserves and certainly of what period of detention will best promote his rehabilitation.”