"Because the brain of a juvenile is not fully developed ... a judge cannot find with confidence that a particular offender, at that point in time, is irretrievably depraved," the court wrote.
Plymouth County District Attorney Timothy J. Cruz said he was "shocked" by the Parole Board's decision, arguing that Christian conspired to commit the murders.
Cruz said Christian has had 31 disciplinary reports—the most recent being in 2012.
"It is clear the defendant didn’t change his conduct until he had something to gain," Cruz said in a statement.
Thursday's ruling could also affect the case of Philip Chism, the 14-year-old Danvers native charged with raping and killing his high school teacher last year. Do you think teenagers should be exempt from life sentences? Share your opinion in the comments.