The National Standards for Youth Justice Services has guidelines for Section 90/91.

Section 90
If the conviction is for murder, the sentence falls under Section 90 of the Powers of the Criminal Courts (Sentencing) Act 2000. Such sentences are termed "Detention at Her Majesty's Pleasure" and a mandatory life sentence will apply.

Section 91
If a young person is convicted of an offence for which an adult could receive at least 14 years in custody, they may be sentenced under Section 91 of the Powers of the Criminal Courts (Sentencing) Act 2000. The length of the sentence can be anywhere up to the adult maximum for the same offence, which for certain offences may be life..

For more details go to the Youth Justice Board website