David Bagshaw was four days from his 18th birthday when he stabbed Stefanie Rengel to death at the urging of his jealous 15-year-old girlfriend.

A young man who pleaded guilty to stabbing a 14-year-old girl to death in exchange for sex with his girlfriend has had his bid to be sentenced as a youth denied.

The Court of Appeal for Ontario on Wednesday quashed an appeal of a life sentence by David Bagshaw, who pleaded guilty to stabbing Toronto teen Stefanie Rengel on New Year's Day in 2008.

He was just four days shy of his 18th birthday at the time, but received an adult sentence of life in prison with no chance of parole for 10 years.

His lawyers had said he should have received a youth sentence of 10 years.

Melissa Todorovic, his 15-year-old girlfriend at the time, was sentenced as an adult to life with no chance of parole for seven years — the maximum adult sentence for someone her age.

Superior Court Justice Ian Nordheimer found during Bagshaw's trial that the young man and Todorovic had a sexual relationship "marked by mutual obsession and jealousy," which culminated in Bagshaw killing Rengel after months of pressure from Todorovic, who mistakenly saw Rengel as her rival.

Todorovic had been "hounding and manipulating" Bagshaw to kill Rengel for months, threatening to withhold sex, end the relationship, have sex with another boy that Bagshaw knew or kill herself unless he went through with the murder, the court heard.