“The principle is simple. Where the offender’s age, as it affects his culpability and the seriousness of the crime justifies it, a substantial, or even a very substantial discount, from the starting point may be appropriate. One way in which the judge may check that the discount is proportionate would be for him to consider it in the context of the overall statutory framework, as if Sch.21 envisaged a flexible starting point for offenders between 18 and 21. This would have the advantage of linking the mitigation which would normally arise from the offender’s relative youth with the statutory provisions which apply to an offender a year or two older, or younger, and would contribute to a desirable level of sentencing consistency. Due allowance should then be made for the relevant aggravating and mitigating features to produce the final determination of the minimum term, and thereafter the judge should explain the reasons for the determination” (Emphasis provided)