Related Rule
Denmark
Practice Relating to Rule 137. Participation of Child Soldiers in Hostilities
Denmark’s Military Criminal Code (1973), as amended in 1978, provides:
Any person who uses war instruments or procedures the application of which violates an international agreement entered into by Denmark or the general rules of international law, shall be liable to the same penalty [i.e. a fine, lenient imprisonment or up to 12 years’ imprisonment]. 
Denmark, Military Criminal Code, 1973, as amended in 1978, § 25(1).
Denmark’s Military Criminal Code (2005) provides:
Any person who deliberately uses war means [“krigsmiddel”] or procedures the application of which violates an international agreement entered into by Denmark or international customary law, shall be liable to the same penalty [i.e. imprisonment up to life imprisonment]. 
Denmark, Military Criminal Code, 2005, § 36(2).
At the 27th International Conference of the Red Cross and Red Crescent in 1999, Denmark pledged “to promote the adoption of national and international standards prohibiting the military … participation in armed conflicts of persons under 18 years of age”. 
Denmark, Pledge made at the 27th International Conference of the Red Cross and Red Crescent, Geneva, 31 October–6 November 1999.
In 2009, in a statement before the UN Security Council on children and armed conflict, made on behalf of Denmark, Finland, Iceland, Norway and Sweden, the permanent representative of Finland stated:
The SC [Security Council] should refer violations against children in armed conflict to the ICC [International Criminal Court] for investigation and prosecution when national governments fail to carry out their responsibility to protect these children. 
Denmark, Statement by the permanent representative of Finland before the UN Security Council on “Children and Armed Conflict” made on behalf of Denmark, Finland, Iceland, Norway and Sweden, 29 April 2009.