A 14-year-old boy has been given the maximum sentence available to a young offender for sexual assaulting a young boy living with him in foster care.

He will serve his time in a group home.

"This is a very difficult case right from the beginning and very disturbing circumstances," said Det. Stephen Johnston of the Calgary Police child abuse unit.

But Johnston says six months of strict regulations and 18 months of continued supervision is the right sentence.

The boy was arrested after neighbours saw him through their window assaulting a four-year-old boy in a southwest foster home.

The older boy has been diagnosed with Fetal Alcohol Syndrome.

Experts testified he has the mental capacity of a six-year-old and has no impulse control, requiring constant supervision.

Both crown and defence lawyers agreed he will be at risk of committing harm or being harmed himself in the young offender's centre.

Judge Lynn Cook-Stanhope says from years of experience she knows the young victim has been badly harmed.

She said once again the criminal court system is left to deal with the scourge of FAS, which she says is entirely preventable.