This section applies where a person aged at least 18 but under 21 is convicted of an offence for which the sentence is not fixed by law.
The court must sentence the offender to custody for life if—
the offence is punishable in the case of a person aged 21 or over with imprisonment for life, and the court considers that a sentence for life would be appropriate, or
the court is required by section 273 , 274 or 274A to impose a sentence of custody for life.
Sections 230 (threshold for imposing discretionary custodial sentence) and 231 (length of discretionary custodial sentences: general provision), in particular, apply for the purposes of subsection (2)(a).