20. Extended sentences for persons aged under 18.

The court1 may impose an extended sentence of detention where2:


    (1)     a person aged under 183 is convicted of a specified offence (whatever the date on which the offence was committed)4;


    (2)     the court considers that there is a significant risk to members of the public of serious harm5 occasioned by the commission by the offender of further specified offences