Britain’s youngest knife murderers, who killed a stranger in a brutal machete attack when they were aged just 12, have both been handed life sentences with a minimum term of eight-and-a-half years.

The killers, both from Wolverhampton, were convicted in June of murdering 19-year-old Shawn Seesahai, who was stabbed in the heart and suffered a skull fracture on the city’s Stowlawn playing fields on November 13 last year.

They were described at Nottingham Crown Court on Thursday as “the youngest knife murderers” and are believed to be the youngest defendants convicted of murder in the UK since Robert Thompson and Jon Venables, both aged 11, were found guilty in 1993 of killing two-year-old James Bulger.

Mr Seesahai’s killers, both now 13, cannot be named because of a court anonymity order.

Both youths were allowed to leave the dock and sit in the back row of the court benches on Friday as High Court judge Mrs Justice Tipples began her sentencing by acknowledging the sentence being passed would be no comfort to the victim’s family.

The judge told the boys: “When you killed Shawn he was 19, starting out in his adult life with everything to live for.

“His parents have lost their son. His sister has lost her brother.

“What you did is horrific and shocking. You did not know Shawn, he was a stranger to you. You both killed Shawn in an attack that lasted less than a minute when he asked you to move (from a bench).

“I am sure you intended to kill him.”

The judge added she could not be sure which of the boys had inflicted a 23cm-deep wound which almost passed all the way through Mr Seesahai’s body.

In a victim impact statement read to the sentencing hearing, the family of Mr Seesahai said they are haunted by thoughts of how scared he must have been when he was killed.

Relatives of Anguilla-born Mr Seesahai described his murder as tragic, unexpected and senseless, and having been committed “for no reason at all”.

Both boys blamed the other for inflicting four wounds with the machete, after a dispute with the victim about sitting on a park bench.

One of the boys admitted possession of the knife prior to the trial, while the other was found guilty of the same charge when they were both unanimously convicted of murder on June 10.

Explaining her reasons for the length of the minimum terms after the boys were taken down to begin their sentences of detention, High Court judge Mrs Justice Tipples said the murder was aggravated by the fact it was an attack involving two offenders.

Mitigating factors included the fact the “spur-of-the-moment attack” was not premeditated, and the young age of the defendants, who were told they will remain in separate secure units where they were held on remand during their trial.

The “extremely vulnerable” first defendant, who admitted buying the murder weapon from a friend for £40 around a month before the attack, had been “groomed and exploited” by others, the court heard.

The judge said the first defendant had had “very many adverse childhood experiences”, including falling victim to exploitation by criminals and “multiple traumas” in childhood for which he was not responsible.

“I agree with the authors of the (pre-sentence) report that he does not at this stage have the maturity to fully appreciate the consequences of his actions,” the judge added.

The court heard the second defendant had a supportive and loving relationship with his parents and was not previously known to the police.

In a statement issued on their behalf by the UK representative of Anguilla, Dorothea Hodge, the family of Mr Seesahai said: “Whilst the family recognise that today justice has been served in respect of the horrific murder of their beloved son Shawn, they are disappointed with the sentence.

“No one hearing the detail of this case could have anything other than horror at the violence that took place that evening.”

West Midlands Police Chief Superintendent Kim Madill, speaking outside court after the sentencing, said: “Shawn was only 19 when his life was taken at the hands of two boys, then aged just 12, who had armed themselves with a machete.

“That reality has had a huge impact on us all, it is both shocking and saddening. The impact of knife crime is devastating no matter where you live in the country, this is an issue that affects us all.”

A Crown Prosecution Service senior Crown prosecutor said the two killers “should have been enjoying their childhood rather than arming themselves with a machete and killing an innocent person”.

Jonathan Roe said: “As prosecutors, we often deal with harrowing cases, but this case is particularly distressing due to the complete senselessness and devastating consequences of the defendants’ actions.

“The defendants at the age of 12 should have been enjoying their childhood rather than arming themselves with a machete and killing an innocent person.

“Shawn Seesahai lost his life in a horrifically cruel way.

“I hope today’s sentencing serves as a reminder of the dangers of carrying machetes.

“Shawn’s family have shown remarkable strength and dignity in the aftermath of such a tragedy and our thoughts are with them at this difficult time.”