At Truro Crown Court on Wednesday, a jury convicted him of seven counts of raping the girl, one of assaulting her by penetration and two of sexually assaulting her.
The court previously heard the girl came forward about her ordeal after lessons in school about healthy relationships.
She spoke to a school counsellor, which led to her mother being called into the school and the matter being reported to the police.
“It was through that process that [she] found the strength to tell an adult about what had happened to her,” said Emily Pitts, for the Crown Prosecution Service, who added that the girl had also told a friend.
The court heard Robinson made the girl perform oral sex on him and had tried to take her virginity when she was aged 12 but stopped because it was hurting her.
Ms Pitts said: “She assumed it was normal; she did not know any different.”
After the verdicts, Bathsheba Cassel, for the defence, said Robinson, of Eddystone Road, was essentially a man of good character, having only one previous conviction for burglary.
She said: “This behaviour stopped not by the reporting of the matter to the authorities but at a much earlier stage.”
Ms Cassel added that Robinson, who suffered from a number of health conditions including a stomach ulcer and arthritis, had always been a working man and also volunteered in his community.
He was currently a National Trust volunteer.
Sentencing him to 22 years in prison, Judge Simon Carr, said anyone who had heard the girl give evidence would not have doubted the truth of her account.
He said: “I am perfectly clear that [the girl] will live with the consequences of what you did for the rest of her life.”
He added that the fact she had had the courage to speak out showed her strength of character – almost the direct opposite of the character Robinson himself possessed.