A sentence of detention in a young offender institution is available to a court dealing with an offender for an offence where—
the offender is aged at least 18 but under 21 when convicted,
the offence is punishable by that court with imprisonment in the case of a person aged 21 or over, and
the court is not required to pass a sentence of—
detention during Her Majesty's pleasure (see section 259), or
custody for life (see sections 272 and 275).
Where—
a sentence of detention in a young offender institution is available, and
the court is not required to impose such a sentence,
the power of the court to impose such a sentence is subject (in particular) to section 230 (threshold for imposing discretionary custodial sentence).
For circumstances in which a court is required to impose a sentence of detention in a young offender institution, see the provisions mentioned in—
section 399(ba) (serious terrorism sentences);
section 399(c) (mandatory minimum sentences).