A slight correction to the observation below: The second treaty that Kennedy cites is not the ICC treaty, but the International Covenant on Civil and Political Rights (ICCPR). This treaty was ratified by the U.S., but with the following reservation:

the United States reserves the right, subject to its Constitutional constraints, to impose capital punishment on any person (other than a pregnant woman) duly convicted under existing or future laws permitting the imposition of capital punishment, including such punishment for crimes committed by persons below eighteen years of age.

In other words, the reservation nullified the specific claim at issue — that U.S. participation in the ICCPR suggests the existence of a “national consensus” against the death penalty for juveniles. Justice Kennedy shrugs this off noting that the reservation was in 1992, and much has changed since then. (No, really, that’s his argument.)