Joshua John Sullivan, 19, pleaded guilty to eight charges of aggravated robbery, attempted aggravated robbery, riding in a stolen car and damaging the vehicle by fire.

The ACT Supreme Court heard Sullivan was part of a group of under-age boys who robbed four victims at knifepoint at the Weston Creek skate park, threatening to slit their throats, in December last year.

They also tried to rob another pair at the Phillip skate park a short time later.

Sullivan was on bail for those offences in November last year when he joined another group of under-age boys who performed burnouts in a stolen car outside Bimberi youth detention centre.

Acting Justice John Nield said Sullivan had ''wasted his life'' for the past few years.

The court heard Sullivan and four friends went to the Weston Creek skate park and struck up a conversation with four young men riding bicycles at the skate park.

One of the under-age boys had a knife in his trousers while Sullivan and another boy had bottles of beer. The boy with the knife asked the four boys, ''Youse got any money?'' but when they said they had none, he pulled the knife out of his pocket and demanded their wallets while Sullivan smashed a beer bottle on to ground.

Acting Justice Nield said: ''I do not doubt that the offender throwing the bottle on the ground and removing the knife were designed to intimidate.''

The boy with the knife then told one of the victims: ''I will slit your throat if you don't give me any [money].''

The court heard Sullivan and his mates took a wallet and mobile phones from the victims. The group moved on to the Phillip skate park, where the boy with the knife tried to rob two young men on rollerblades, threatening to stab them if they did not hand over their wallets and phones.

But the two victims managed to flee, jumping into their cars with Sullivan and the boy with the knife in pursuit.

Sullivan was granted bail over the robberies but last November he was a passenger in the stolen car doing burnouts outside Bimberi. He was with four under-age boys at the time.

The court heard the car broke down and the group pushed it about 100 metres along Old Wells Station Road but it stopped again. They then jumped on the car roof, broke its windows and set fire to the vehicle before police chased them into the Canberra racecourse, where they were arrested.

Sullivan was sentenced to four years and three months in jail, back-dated to account for time already served. He will be eligible for parole in November next year.