Thompson V. Oklahoma from 1988: "...The conclusion that it would offend civilized standards of decency to execute a person who was less than 16 years old at the time of his or her offense is consistent with the views that have been expressed by respected professional organizations, by other nations that share our Anglo-American heritage, and by the leading members of the Western European community. ... The death penalty has been abolished in West Germany, France, Portugal, The Netherlands, and all of the Scandinavian countries, and is available only for exceptional crimes such as treason in Canada, Italy, Spain, and Switzerland. Juvenile executions are also prohibited in the Soviet Union."
But hwether we adopt other countries' views is up to the legislature. As is responding to opinion polls.
The Court does not see the ironic futility of an unelected tribunal trying to be the "engine of democracy"!