Even though I haven't yet read this opinion, I predict that given that the US is the only Western Nation that still allows for [or allowed for] executions of people who committed crimes as minors, the opinion will cite such foreign law and this will drive the right-wingers nuts.
Personally, if a near adult commits a capital crime, and if capital punishment is legit, I don't see why not. Given that it takes about 20-years to execute someone, it's not like we are executing them when they are minors.
My only reservation about the DP is putting an innocent to death, which, as far as I know, has never happened in the modern era (at least there's no proof of it). And the reason why it's never happened is because we execute so damn few people and go through an infinite process of i dotting and t crossing that it's practically impossible for an innocent to be put to death. But that in turn transforms the death penalty into this expensive, arbitrary process where 20,000 homicides occur every year but less than 100 executions.
But when we have a "no-doubt" case, for instance, the BTK serial killer who admitted to doing the murders, I see no reason to oppose the execution and I look forward to the day that he gets it.
Update: My Dad writes:
"I'm not so sure that no innocent individuals have been executed. It seems that there was a very questionable execution in VA, and perhaps others."
"No death penalty for the BTK killer as it was not reinstated in Kansas until after all of the killings occurred. Also, I do believe that Kennedy cited foreign sources in his opinion and Scalia was his ornery self with his dissent."