The decision means that John Lee Malvo would be eligible for the death penalty if he were found guilty.
Mr Malvo is to be tried - probably in autumn - in the state of Virginia, which has the second highest rate of executions in the country.
Along with John Allen Muhammad, he is accused of killing 10 people and wounding three others.
The decision was based on evidence that the juvenile, John Lee Malvo, was involved in at least some of the killings last year and that he tried to extort $10m from the authorities.
The most dramatic evidence came from William Franklin, husband of one of the victims.
Horrific account
He told the court that as he and his wife were loading their car last October, he heard a noise and something hit the side of his face.
He turned round to see his wife lying on the ground.
He later found his face splattered with her blood.
Law enforcement officials also told the court that Mr Malvo's fingerprints and handwriting link him to at least four of the 10 people allegedly killed by the snipers.
Prosecutors were able to convince the judge there was enough evidence to try Mr Malvo as an adult - which allows them to push for the death penalty if the teenager is convicted.
John Lee Malvo's alleged accomplice, 42-year-old John Allen Mohammed already faces execution for his suspected role in the sniper killings.