Teenager, 15, admits to rape, assault and robbery of grandmother in Perth

The 15-year-old. Photo / Supplied

A 15-year-old boy has admitted to a sex attack on an 83-year-old grandmother that included forcing her to shower to wash the evidence away.

The attack was so viscous it left the woman severely injured and suffering trauma.

The 15-year-old, who cannot be named, pleaded guilty to sexual assault, robbery, and the bashing of the woman after breaking into her home at 5am on a Friday morning in Waikiki, an outer southern suburb of Perth.

He is alleged to have lived in the area and it's believed the woman was asleep when he broke in on October 13 and pinned her to the floor.

Police released CCTV footage showing a hooded figure exiting the victim's home.

Details in court and reported by WA Today reveal the brutality of the attack, in which the boy "sexually penetrated her three times during the home invasion".

Appearing via video link from Banksia Hill Detention Centre, the boy bit his nails as charges were read in Perth Children's Court.

According to ABC News the woman alerted neighbours after the attack.

After the footage went public the teenager was located by detectives from the WA Police Sexual Assault Squad two days later.

9News reported the grandmother was robbed, raped and forced to shower by her attacker. He also stole the woman's handbag.

The Daily Mail reports the boy was on parole at the time of the attack.

"He was just normal, quite like how he his all the time," an older cousin said.

"He's just in with the wrong crowd."

The victim was rushed to hospital and was later released.

The boy was remanded in custody and will be sentenced at a later date, after a neuropsychological report is undertaken. He will also be tested for foetal alcohol syndrome.