I think that the fact he has a subsequent conviction for rape, should have superceded the trying him as a youth offence thing.
I can understand them saying - yes he did this when he was X so we need to look at it as then and sentence as we would have had then.
BUT in the meantime this behaviour has been shown to be a pattern
AND presumably he would have got longer in prison for the later rape, if these offences had been tried and convicted at the time.
So actually, logically, it doesn't make sense, what they have done. Due to the subsequent behaviour and conviction.