It is "inexplicable" that a teenager, who prosecutors want declared a dangerous sex offender, has not been given the opportunity to properly rehabilitate himself while in custody, a Perth judge says.

The 19-year-old, who can not be identified, started sexually offending when he was about 14, raping a drunk 12-year-old girl in WA's north.

His latest offence occurred in 2014, when he and another offender repeatedly punched a 24-year-old woman whom he also tried to sexually assault.

He was sentenced to three years in jail for that offence and despite being eligible for parole he asked not to be released from prison.

Prosecutors are now seeking to have him declared a Dangerous Sex Offender (DSO), and have applied to the Supreme Court for a hearing to be conducted.

The offender, and his lawyers, have opposed the application on the grounds of the teenager's youth, and because of what they claimed was the state's "inaction" in meeting his "treatment needs" whilst he was in custody.

Justice Jenkins said the teenager had no experience dealing with regular daily tasks.

Justice Lindy Jenkins has now ruled that a hearing should be conducted to determine if the teenager should be subject to a DSO order.

But in her decision, Justice Jenkins said it was "disappointing" that previous recommendations about the teenager had not been implemented, in particular a recommendation that he be medically assessed for Foetal Alcohol Spectrum Disorder (FASD).

The court was told the teenager's mother abused alcohol while she was pregnant, and during his childhood he was exposed to domestic violence, substance abuse, inappropriate sexual activity, and the suicide of his nine-year-old "cultural brother".

Justice Jenkins said the 19-year-old also had problems with literacy, numeracy and language, and had no experience with daily living tasks, such as shopping or managing public transport.

She also found he could not "demonstrate any understanding of employment processes ... or the need to prepare for a working life".

"On evidence before me, it is inexplicable that [the man] has not been given the opportunity to address the causes of his offending behaviour in a formal and structured manner, whilst he has been on juvenile community-based orders, in juvenile detention and in adult custody," she said.

However, Justice Jenkins said her role was only to determine if there were reasonable grounds for believing a court might find that the man is a serious danger to the community, an outcome she considered likely.

She ordered that in preparation for the hearing, the man undergo assessment for FASD, that he be referred to a speech pathologist, and that he have a hearing test.