The 16-year-old girl, who cannot be named, was instead sentenced to an 18-month youth justice supervision order for the February 27 incident.
The teen admitted she threatened a taxi driver with a knife before stealing his car about two hours after the "adult time" legislation came into effect.
Under the changes, children aged 14 and older who commit home invasions or aggravated carjackings must face the County Court, where penalties are tougher than a children's court.
Victorian County Court Judge Scott Johns found the offending was "brazen and violent".
But her upbringing and the absence of any criminal record meant she should be sentenced under the children's act.
"Specific deterrence and protection of the community would be best served by your rehabilitation in the community," the judge said on Wednesday.
"No doubt for many cases of aggravated carjacking, sentences imposed under the (adult) Sentencing Act will be necessary.
"I've concluded your case is not one of them."
The balaclava-clad girl and her male co-offender jumped into the back of the victim's taxi about 2.47am on February 27, the court was told.
After the driver told them to leave, the girl pointed a knife at his neck and demanded he get out of the car and leave his phone and keys behind.
Fearing he would be stabbed, the driver stepped out and called for help as the girl drove the car away from Dandenong train station in Melbourne's southeast.
The boy was in the front passenger seat.
The pair crashed a short time later after speeding along the Princes Highway at 180km/h.
The boy was in March sentenced in a children's court to a 12-month good behaviour bond after pleading guilty to carjacking.
But the girl faced the higher court after pleading guilty to the more serious aggravated carjacking charge.
In delivering his sentence, Judge Johns said the offence was inherently serious and the victim had been a vulnerable, unsuspecting target.
"He was in his workplace and he was entitled to be safe," the judge said.
He noted it was her first time before the courts and she had a difficult upbringing, having been in and out of child protection and residential care since the age of two.
Mental health conditions also affected her ability to reason, he said.
Her supervision order includes conditions requiring mental health and illicit substances treatment, participation in a day program like school, and a curfew between midnight and 6am.
The judge warned not complying with the "onerous" conditions would result in her being re-sentenced.
"You haven't had a lot of opportunities in your young life but this is one," the judge told the girl.
"Good luck. I hope I don't see you back here."
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