Human Rights Law
International law prohibits life without parole sentences for those who commit their crimes before the age of 18.
It is a prohibition that is universally applied outside of the United States. Oversight and enforcement bodies for two treaties to which the United States is a party (the International Covenant on Civil and Political Rights and the International Convention on the Elimination of All Forms of Racial Discrimination) have found the practice of sentencing juvenile offenders to life without parole to be a clear violation of US treaty obligations.
Read International Human Rights Law (PDF) for excerpts from several treaties and findings of oversight and enforcement bodies.