相关规则
Japan
Practice Relating to Rule 120. Accommodation for Children Deprived of Their Liberty
Upon ratification of the 1989 Convention on the Rights of the Child, Japan reserved the right not to apply Article 37(c) “considering the fact that in Japan as regards persons deprived of liberty, those who are below 20 years of age are to be generally separated from those who are of 20 years of age and over under its national law”. 
Japan, Reservations and declarations made upon ratification of the Convention on the Rights of the Child, 22 April 1994, reprinted in UN Doc. CRC/C/2/Rev.4, 28 July 1995, p. 22.
In 2007, in its fifth periodic report to the Human Rights Committee, Japan stated:
Sentenced juvenile inmates are detained in penal institutions which have been established especially for juveniles, such as juvenile prisons or special areas which have been established within penal institutions, and thereby, are separated from adult sentenced inmates. 
Japan, Fifth periodic report to the Human Rights Committee, UN Doc. CCPR/C/JPN/5, 23 April 2007, submitted 20 December 2006, § 213.