Patrick Curran killed 74-year-old neighbour Joan Roddam at her home in Delabole, Cornwall, in November 2003 when aged 27, Truro Crown Court heard.
In the days before the killing, he had been to the doctor and reported a raised libido.
Mr Curran was arrested in 2014 when the case was re-examined. He denies murder.
The jury heard that Mrs Roddam's body was discovered in a field behind her bungalow in West Down Road on 8 November 2003 after a friend was unable to contact her.
Mr Curran, now aged 38, had lived with his parents at the time of the killing, the jury heard.
The court heard that in the days before Mrs Roddam was killed, Mr Curran had been suffering from a form of mental illness and had visited a doctor and reported a raised libido.
The prosecution said the defendant had a sexual interest in woman in their 70s and older and had visited pornographic websites that featured women very much older than him.
The court heard Mr Curran had not planned to kill Mrs Roddam when he visited her bungalow but attacked her when she rebuffed his sexual advances.
The defendant, of West Down Road, was arrested last year after new forensic evidence was produced, the court was told.
The trial continues.
A man with a sexual interest in much older women strangled a pensioner when she rebuffed his sexual advances, a court has been told.