At the court in Manchester, Justice John Saunders said it was “chilling” that the teen, who cannot be named under British law, had been radicalized at such a young age.
The sentencing comes at a time when U.K. authorities are concerned about the risk of so-called “lone wolf” attacks, not only by foreigners but also by Britons radicalized in part by extremist material online.
The court had previously heard that the plot was in the advanced stages. In March, the British teen had exchanged more than 3,000 messages with Sevdet Besim, an 18-year-old Australian whom prosecutors argued used the online name "Illyas." In one exchange, they discussed a plan to run over a police officer at the Anzac Day parade.
In another, the British teen suggested that the Australian should “break into someone’s house and get your first taste of beheading” before the parade.
In July, the teen pleaded guilty to inciting an act of terrorism overseas. Now 15, he must serve a minimum of five years in prison and will be released only if he is deemed not to be dangerous, the judge said.
The British teen, from Blackburn in northwest England, was first arrested in March over suspicion of threatening to kill his teachers. But when police examined his cellphone, they found a screensaver of Islamic State militants and messages about a terrorist plot in Australia. The Greater Manchester Police informed police in Australia, who arrested five men in April, a week before the parade. One of the men, Besim, has been charged with conspiring to commit terrorist acts and is awaiting trial.
The court heard that the Briton was struggling with school, home life and a degenerative eye condition, and that an online jihadist community had “filled a void” in his life, according to the BBC. Within a couple of weeks of setting up a Twitter account, he had amassed 24,000 followers.
His lawyer, Daniel King, told reporters that the teenager’s family was “completely unaware of his activities” and “relieved that no one was injured” as a result of his actions.
"People will understandably be shocked by the age of the boy,” Tony Mole, detective chief superintendent at Greater Manchester Police, said in a statement. “However this should not detract from the horror of what he was planning. It is also a clear message that you will face prosecution, no matter how old you are.”