"What good can come of raking over old sores?"
Well, a bit of justice for the victims of crime and, in this case, the relatives. Would you suggest that the whole Savile thing shouldn't be investigated because it was a long time ago? Or that Stephen Lawrence's killers shouldn't be pursued?
PC Blakelock was based in the police station at the end of my road and, while I didn't know him, his face was familiar to all in the area. The memorial to him was placed in Muswell Hill Broadway because to put it where he died would have inflamed the situation. Folk put flowers in it to this day. So yes, if they can prove it - and I hope they can, of course the suspect should be put on trial.
To answer another question, as he was a minor when the alleged offence was committed, I think he'd be tried now as an adult, but that the sentence would reflect his age at the time of the offence.