Iran has hanged a man for a crime committed when he was seventeen years of age: http://www.upi.com/Top_News/International/2009/10/13/Iran-hangs-man-sentenced-when-17/UPI-23491255453386/. This violates Iran's obligations under article 6(5) of the International Covenant on Civil and Political Rights, article 37(a) of the Convention on the Rights of the Child and under customary international law as codified in the Safeguards adopted by the Economic and Social Council in the early 1980s.
Iran is probably the only country in the world to persist in such violations. The United States of America ceased executions for juvenile offences in 2005, following a ruling of the Supreme Court.
The prohibition on executions for juvenile crimes has become a universal norm since it was first set out in the fourth Geneva Convention in 1949. It isn't the only problem Iran has with modern human rights standards, but it is certainly a flagrant one.