Daniel O'Connor, now 19, allegedly entered the woman's bungalow in Basingstoke, Hants, in December 2003.
The woman, who cannot be named for legal reasons, initially did not report the attack because she felt "ashamed".
Mr O'Connor, formerly of Ovington, Hampshire, denies indecent assault and burglary with intent to commit rape.
"She was not going to report it initially. She felt very ashamed about what had happened "
Susan Evans, prosecuting, told the court he had pulled the woman's personal alarm from her neck before attacking her for about 20 minutes.
She said the woman could not see properly because she did not have her glasses on but later saw a knife on the floor which she said was taken from the kitchen.
Miss Evans added: "She was not going to report it initially. She felt very ashamed about what had happened and went to her bedroom and went to sleep."
The woman later told her daughter and her carer and the police and paramedics were then called.
'Fingerprints found'
She told detectives she had scratched her attacker and, after Mr O'Connor was arrested last October, his DNA was matched with that taken from skin found under the woman's nails, the court heard.
His fingerprints were also found in her one-bedroom bungalow, the prosecution said.
When he was interviewed by police Mr O'Connor said he had never been "anywhere near" the woman's home.
The trial continues.
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