Ryan Herbert was aged 16 when he was jailed for life in 2008 for murdering Sophie Lancaster in a park in Bacup, Lancashire.
The Parole Board decided Herbert can now be released on licence.
Reacting to the decision, Sophie’s mother, Sylvia Lancaster, told BBC News: “Once again we have a justice system that fails to deliver justice.”
She said the original sentence reflected the “level of violence” her daughter was subjected to during a “feral” attack.
Herbert, whose minimum term was cut in 2020, and a friend battered Sophie as she cradled her boyfriend’s head, who they had also attacked.
She died 13 days after being beaten by Herbert and Brendan Harris as she tried to protect Robert Maltby from an attack in Stubbylee Park.
Herbert, of Bacup, admitted both murder and assault causing grievous bodily harm with intent and was given a minimum term of 16 years and three months, which was later reduced to 15 and a half years.
He had his minimum tariff reduced to 14 and a half years in February 2020, making him now eligible for parole consideration two years later.
High Court judge Mr Justice William Davis concluded the tariff could be reduced as he had made “exceptional progress” in prison.
Ms Lancaster said she could not understand the decision being made “because they have done well in prison” when the Parole Board knew “the reality of the level of violence” Sophie had been subjected to.
“However much progress people make in prison, they have been given their sentence in recognition of the extent of their crime,” she said.
Ms Lancaster said her daughter’s injuries were “so severe” paramedics could not “facially distinguish if she were male or female”.
The judge described the attack as “feral” and her attackers as “savage and merciless”, she added.
The Parole Board’s ruling found Herbert had made “significant changes to his life which reflected his remorse, his insight and increased maturity”.
“After considering the circumstances of his offending and examining the evidence for the progress made while in custody, the panel was satisfied that Mr Herbert was suitable for release,” a document setting out the decision said.
The document said Herbert’s behaviour had “initially been poor” but this changed once he moved into an adult prison and he had taken part in rehabilitation programmes.
After being moved into an open prison in November 2020, he “improved his education with studies to degree level” and had “fully engaged with resettlement activities” including spending time on temporary release from jail, it added.
“No concerns about compliance had been identified and Mr Herbert had good working relationships with professional staff,” it said.
Witnesses, including his probation officer, recommended he be released on licence.
Herbert will be subject to restrictions on his movements, where he lives and who he contacts.
Harris, who was found guilty of Sophie’s murder in 2008 and admitted the attack on Mr Maltby, was jailed for life with a minimum term of 18 years.