The 17-year-old offender faced the ACT Supreme Court on Friday afternoon for a sentence hearing, having pleaded guilty in April to a charge of recklessly inflicting grievous bodily harm.

He was just 15 when he stabbed a 16-year-old boy in the back during a deadly brawl at the Weston Creek skatepark, where 12 people gathered to fight following a war of words on social media.

None of those involved in the September 2020 incident can be named for legal reasons.

While the 17-year-old boy admitted the non-fatal knife attack, which left the victim seriously injured and permanently scarred, he pleaded not guilty to a murder charge stemming from the same brawl.

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He argued he was not responsible for knifing his victim's 18-year-old cousin, who died at the scene after suffering six stab wounds, and a jury found him not guilty of the homicide after a five-week trial.

The boy took the witness stand in his own defence, saying he had picked up the knife he used to stab the younger cousin from the ground after it had presumably been used by someone else to kill the deceased.

His barrister, David Barrow, said on Friday that the boy had suffered from an attention deficit disorder from an early age, predisposing the child to impulsive and reckless behaviour.

He agreed with Chief Justice Lucy McCallum that the offender's youth also contributed to such actions, adding that the condition made the boy "even more ill-equipped ... than an ordinary teenager" to deal with the "volatile" situation at the skatepark.

Mr Barrow ultimately said "substantial punishment has been meted out already", citing his client having spent several months behind bars on remand, among other things, as he argued a good behaviour order with supervision was the most suitable way forward.

Crown prosecutor Rebecca Christensen SC argued further jail time, albeit suspended, was warranted to reflect the seriousness of the grievous bodily harm offence and its maximum penalty of 13 years.

She also said a lengthy period of supervision in the community was required, telling the court the boy had a demonstrated history of "acts of escalation to violence".

Chief Justice McCallum said she planned to sentence the boy on August 9.

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