The case of a young offender who pleaded guilty to sexually assaulting a four-year-old in a foster home where they both lived will proceed to sentencing in the new year.

Concerns were raised after one of the doctors assessing the 13-year-old reported the teen had a mental age of six and an IQ of 56.

But another doctor said he had no doubt the teenager, who has Fetal Alcohol Spectrum Disorder, was fit to stand trial.

The judge reviewed reports by other doctors and defined a difference between mental fitness and competence to conclude the teen is fit to stand trial.

In making her deicison Monday, the judge said she is satisfied that whatever the boys' mental capacity "this young person seems well aware of what he did."

Youth court judge Lynn Cook-Stanhope says she has little choice but to proceed with sentencing in this case, but she does so with a "great deal of disquiet."

"Whatever the IQ of this individual, it's clear the test for [mental] fitness in Canada has a very low threshold indeed," said Cook-Stanhope.

Sentencing arguments set for January

She added that youth suffering from FASD are not well served by Canada's justice system.

A date for sentencing submissions has been set for January.

The boy, who cannot be named, had originally been charged with three counts of sexual assault and three counts of sexual interference after police were called to a southwest home in January when a third party said they had witnessed an assault.

The alleged victims were aged six, four and two and all the boys were residents of the foster home.

The Crown went ahead with the charge involving the four-year-old boy. In March, the teen pleaded guilty to that sexual assault.

But court proceedings were interrupted in September after Cook-Stanhope raised "grave concerns" about the 13-year-old's mental capacity.