Well we know some things about this now. We know that the pre-frontal cortex of the brain, the bit responsible for self-control; call it adult behaviour, is still developing until the age of 24ish. we also know that people - mostly boys - that get involved with the law whilst young, peak their criminal behaviour at around the same time and all but about 10% eventually give it up. In fact age is the best crime prevention method.
Also, when boys get long term relationships and jobs their offending declines rapidly.
So, in summary, anything we can do to get young offenders older-headed quicker - like getting a job - we should do, and writing off convictions from a child's record seems very sensible - for once.