James Trewick, who had worked for the Red Cross and Age Concern, targeted two girls who were too afraid to report what he had done to them.
Newcastle Crown Court heard his crimes had a devastating affect on the victims, who had to relive their ordeals by giving evidence against the now 66-year-old at trial.
One said in an impact statement she has suffered "over 40 years of not feeling normal", feels "anger I didn't scream at the time" and said: "For years I have had to pretend everything was normal.”
She added: "I'm angry I didn't feel strong enough to speak out and even as an adult I kept this secret for so long.
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"I still have deep trust issues with people. I also don't want an apology from him because I don't believe it will be genuine.
"He is only sorry he has been exposed, and I will never forgive him."
The other said she has felt "dirty, ashamed and guilty" through her life and added: "I would love to be able to forget about the things I was subjected to, but I can't and don't think I ever will.
"I just can't find it in myself to forgive him, even more so after he put us through a trial."
Trewick, of Wheatridge, Seaton Deleval, admitted four charges of indecent assault.
He was convicted of a further eight offences of indecent assault, as well as two charges of attempted rape.
Judge Robert Adams sentenced him to a total of 13-and-a-half years behind bars, with a two-year extended licence period.
Peter Schofield, mitigating, said Trewick lived a "normal life, without fault or criticism" after the offences and was now a married grandfather who is in poor health.
He also told the court Trewick was a "different man today".
Detective Constable Andrea Nixon praised the victims’ bravery and said she hoped the outcome would urge other victims of non-recent abuse to come forward.
She added: "These victims have shown incredible strength coming forward and while the outcome today will never give them back their childhood, it will hopefully help them feel that some justice has been done.”