I think your best bet is to focus on the idea that due to diminished culpability (because of youth), retribution does not provide sufficient justification of the DP. As an example: we don't allow juveniles to make the momentous (and admirable) decision to enlist in the Armed Forces without parental permission. Thus, the U.S. Congress has made a policy decision that juveniles do not have the mental capacity to make such important decisions. The same can be argued about murder: do juveniles (in general) have sufficient mental capacity to form the requisite intent to commit murder? If the answer is no, then retribution (the primary goal of punishment) is less important because the juvenile could not form the level of intent that we as a society seek to punish.
You forgot the most important point: if we execute murdering sons-a-B*&^%es for murders they commit before turning 18, the French might be inclined to think less of us than they already do.
Ditto Italian Communists and British Labour Party members.
And we wouldn't want that.