One of the provisions of the Convention on the Rights of the Child is that a person under the age of 18 - or else under the age of majority - can not be executed. Two countries - Saudi Arabia, and Sudan - still have the death penalty for juveniles, or, rather have used it since 2009. Iran appears to have abolished it in 2012.
These executions constitute blatantly breaking international law. Here is an excerpt from Wikipedia concerning the Saudi scenario regarding juvenile capital punishment:
The Committee on the Rights of the Child, which reviewed Saudi Arabia's treatment of children under the Convention in January 2005, strongly condemned the government for its practice of imposing the death penalty on juveniles, calling it "a serious violation of the fundamental rights". The committee said it was "deeply alarmed" over the discretionary power judges hold to treat juveniles as adults: In its 2004 report the Saudi Arabian government had stated that it "never imposes capital punishment on persons ... below the age of 18". The government delegation later acknowledged that a judge could impose the death penalty whenever he decided that the convicted person had reached his or her majority, regardless of the person's actual age at the time of the crime or at the time of the scheduled execution.
The last bit is taken from here. It gives one example of criteria used to determine whether or not a person has reached the age of majority (emphasis mine):
In September 2005, Human Rights Watch urged the Saudi government to commute the death sentence of 14-year-old Ahmad D., sentenced to death in July 2005 for killing another child when he was 13. The court in Dammam tried Ahmad as an adult based on its assessment of the coarseness of his voice and the appearance of pubic hair.
How do Saudi judges decide whether or not a person has become an adult and is eligible for the death penalty?