I had thought the various States had a reasonable approach. Those that allowed the death penalty for 16- and 17-year-olds had safeguards in place: first, they had to convince a court to allow the youthful offenders to be tried as adults; second, they had to give notice that they were seeking the death penalty, which places extra burdens upon the prosecution (at least, after a guilty verdict). What these States were attempting to say is, some 16- and 17-year-olds are old enough to be judged as adults, some (most) aren't. That sounds reasonable to me.
But, OK, so the Supremes said No, 18 is the line. Well, fine. But is 18 the line because the Court says that's the actual line of demarcation between adulthood and childhood? Or is 18 the line because that is Society's (and the Law's) best approximation of adulthood? And what happens if we keep lowering the age of adulthood? It used to be 21 for darn near everything. Now it's mostly 18. What if, in future, we all decide that 16-year-olds should be able to vote, graduate HS, enter the military, etc. etc.? Would that automatically mean that the Court should be asked to LOWER the age of liability for capital punishment?
The problem with "progressive" thinking (like that of the libs on the Court) is that it is linear. It only goes one direction. It cannot conceive that a problem will ever be phrased so as to demand a different result. Thomas Cranmer paid the ultimate price for his linear thinking: having led the clergy to w/draw obedience to the Pope as an act of obedience to his king, he found himself trapped when Mary I succeeded to the throne and demanded that he reinstitute obedience to the Pope as an act of obedience to her.
"Progressive" thinking is inherently culture-bound. The only two ways to avoid it are to either 1) state the problem to be solved so that it fits ANY cultural situation from any era, or 2) state the problem so that it fits only OUR historical cultural situation. In other words, either state a universal that will be universally valid (in which case, what's so magic about 18?) OR derive a judgment strictly from American law and jurisprudence.
"Evolving standards" illustrates what G. K. Chesterton was talking about when he said that if you reduce any idea to words of one syllable you can find what they're really talking about. His example had to do with "indeterminate sentencing," which he said meant, I wish Brown to go to gaol and Jones to say when he shall get out. The "progressive" idea thus disguised a hidden tyranny of one person over others. In the case of the current Supreme Court opinion, the "evolving standards" of justice (in words of one syllable) would be something like, We want the law to change when we or our friends think it should, but with no one to ask the folk what they want and with no way to change what we have done.
Now, I'm not arguing for or against the use of the death penalty here. I'm just flummoxed at the illiberal, regressive, autocratic, arrogance of the Supreme Court.