Thorburn, 22, of School House Cottages, Louth, was convicted after a trial of two offences of sexual activity with a girl between June and August last year.
The girl had no previous sexual experience when he took advantage of her, Grimsby Crown Court heard.
Judge David Tremberg told Thorburn: “She had a schoolgirl crush on you and you exploited that and exploited her desire for you.
“Over the course of some weeks, you groomed that young girl, at first treating her like a little sister and then play- fighting with her, telling her she was beautiful, telling her you loved her.”
The pair had sex in his car and, some weeks later, Thorburn sought her out again and had unprotected sex with her, taking the risk of infection and pregnancy.
Judge Tremberg said: “She felt used, as she had been.
“The law is designed to protect precocious young girls from their own instincts and to prevent sexually predatory adults like you from behaving as you have done.
“You put the complainant through the unpleasant experience of coming to court and going through this all again.
“There has been no hint of remorse from you.”
Thorburn, formerly of Tunnard Street and Oxford Street, Grimsby, was ordered to register as a sex offender for life and was banned for life from working with children.
Jeremy Lindsay, mitigating, said Thorburn used no force, coercion, threats, violence or manipulation.
He said Thorburn had drunk a considerable amount of alcohol on the second occasion.
He had no previous convictions and they were isolated incidents. He had been in custody for 38 days.