Were the police to decide not to prosecute, would it be correct to say that the boy could consider a private prosecution when he ceased to be a minor?
Well, he could consider it but, unless he were exceedingly rich, he could never bring the case to trial.
If the Crown Prosecution Service with all its expertise and with all its experience of winning and losing was not willing to progress the complaint at the time the offence was allegedly committed, it’s almost certainly because they didn’t think the chances of a conviction warranted the expenditure of the time and money necessary to test the matter before a jury.