Oh and in terms of where the youngest will go:
The very youngest prisoners, those aged 15, 16 and 17 are held in juvenile units. There are 5 dedicated juvenile establishments. The Youth Justice Board jointly manages the Juvenile estate. Juveniles are held under a Detention and Training Order with a term of 4 - 24 months. Half of this time is served in custody and the remainder under the supervision of a probation officer, social worker or a member of a youth offending team.
Young prisoners 18 - 21 years of age are held in young offender institutions (YOIs). Like training prisons, YOIs can be either 'open' or 'closed'. Young offenders are transferred to adult prisons, before their 22nd birthday, for the remainder of their sentence.
Training prisons are used to hold sentenced prisoners and can be either 'open' or 'closed' depending on the level of security required.
There are separate local prisons for men and women. They accommodate people who need to be kept in custody after being charged with an offence. Such people will generally remain in local prisons until they are released on bail, discharged or sentenced.
We have 17yo are our YOI. Because of their conviction you can assume they will all be at closed prisons at this stage of their sentence too.