s 210B Offenders under 21: power to impose detention in a young offender institution
(1) A sentence of detention in a young offender institution is available to the Court Martial or the Service Civilian Court dealing with an offender for an offence where— (a) the offender is aged at least 18 but under 21 when convicted, (b) the offence is punishable with imprisonment in the case of a person aged 21 or over, and (c) the court is not required to pass a sentence of— (i) detention at Her Majesty's pleasure, or (ii) custody for life. (2) The maximum term of detention in a young offender institution that a court may impose for an offence is the same as the maximum term of imprisonment that it may impose for the offence in the case of a person aged 21 or over. (3) The minimum term of a sentence of detention in a young offender institution is 21 days.