A teenage boy has pleaded guilty to the manslaughter of another teen at a gatecrashed birthday party.

He died three weeks later in hospital, on April 10.

Jason Langhans died at the Alfred Hospital in Melbourne after his family made the heartbreaking decision to turn off life support. ((Nine/Supplied))

Police at the time believed Jason had been hit twice in the head with a screwdriver after stepping in as peacemaker when three people tried to crash the party at Tooradin.

A now-18-year-old boy pleaded guilty to manslaughter in the Victorian Supreme Court on Thursday.

He cannot be identified because he too was 17 at the time.

When asked how he pleaded to the charge of killing Jason, the boy looked at his lawyer before saying “guilty, yeah”.

The teen, who is in custody, will face a pre-sentence hearing in March next year.

Jason was described by family after his death as an outdoorsy boy who loved fishing, camping, four-wheel-driving and playing football.

He had signed up for an apprenticeship, bought a car and “just loved life”, his mother Carolyn said.

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