I'll quote from the dissenting opinion Scalia:
We need not look far to find studies contradicting the Court's conclusions. As petitioner points out, the American Psychological Association (APA), which claims in this case that scientific evidence shows persons under 18 lack the ability to take moral responsibility for their decisions, has previously taken precisely the opposite position before this very Court. In its brief in Hodgson v. Minnesota, 497 U. S. 417 (1990), the APA found a "rich body of research" showing that juveniles are mature enough to decide whether to obtain an abortion without parental involvement. Brief for APA as Amicus Curiae, O. T. 1989, No. 88-805 etc., p. 18. The APA brief, citing psychology treatises and studies too numerous to list here, asserted: "[B]y middle adolescence (age 14-15) young people develop abilities similar to adults in reasoning about moral dilemmas, understanding social rules and laws, [and] reasoning about interpersonal relationships and interpersonal problems." Id., at 19-20 (citations omitted). Given the nuances of scientific methodology and conflicting views, courts--which can only consider the limited evidence on the record before them--are ill equipped to determine which view of science is the right one. Legislatures "are better qualified to weigh and 'evaluate the results of statistical studies in terms of their own local conditions and with a flexibility of approach that is not available to the courts.' " McCleskey v. Kemp, 481 U. S. 279, 319 (1987) (quoting Gregg, supra, at 186).
It is interesting that whereas the Court is not content to accept what the States of our Federal Union say, but insists on inquiring into what they do (specifically, whether they in fact apply the juvenile death penalty that their laws allow), the Court is quite willing to believe that every foreign nation--of whatever tyrannical political makeup and with however subservient or incompetent a court system--in fact adheres to a rule of no death penalty for offenders under 18. Nor does the Court inquire into how many of the countries that have the death penalty, but have forsworn (on paper at least) imposing that penalty on offenders under 18, have what no State of this country can constitutionally have: a mandatory death penalty for certain crimes, with no possibility of mitigation by the sentencing authority, for youth or any other reason
And to tie the second point up with the first :
And let us not forget the Court's abortion jurisprudence, which makes us one of only six countries that allow abortion on demand until the point of viability.
So, underage girls in America can get abortions without their parent's permission, but 17 year old cold-blooded murderers cannot face the death penalty because they are too young to understand their actions, because foreign powers think we're big baddies to have the death penalty in the first place but most of them don't have our liberal abortion rights (and a few of 'em stone to death 14-year-old-girls because they had pre-martial sex).