While Victoria's latest Crime Statistics Agency figures show criminal offences fell 0.2 per cent in the 12 months to the end of March, youth offending dropped by six per cent for the first time in four years.
Victoria Police Deputy Commissioner Bob Hill said he could not put his finger on one issue that had led to the reduction.
"I can't say it's legislation, I can't say whether it's the enforcement activities, the offenders have shifted their thinking, or how they do their business," he said on Thursday.
Asked if youth offenders were ageing out into the adult offenders category, Mr Hill said it was a "fair assumption".
"We've seen a shift in the offending age groups - within the 10 to 17 year olds - where we saw them moving from the 14 to 15 to the 16 to 17... that is one element," he said.
The state government championed its tough reforms as the driving force behind the decline in youth offending.
Under laws announced in November, children who commit violent crimes in Victoria face adult punishment in adult courts, with jail time and longer sentences more likely.
"There's been clearly a concentrated effort around youth crime and making sure that we have a clear demonstration of (policy)," Police Minister Anthony Carbines said.
However, Swinburne criminology expert Joel McGregor said it was too early to tell whether the government's tougher adult time for violent crime reforms, similar to laws enacted in Queensland, had played a role in the drop.
He said no one factor had led to a decrease in offending.
"It reflects a mix in targeted policing, tougher bail settings and also enforcement against repeat offenders, particularly young offenders," Dr McGregor said.
While youth offending was down six per cent, the rate of alleged adult incidents rose by 10.8 per cent in the past year.
The rate of alleged offenders aged 15 decreased by 22 per cent in the same period.
Overall crime in Victoria has increased by 26 per cent in the past three years.
The state opposition said Victoria's crime rates were still higher than when Jacinta Allan became premier in September 2023.
The statistics release comes as NSW grapples with a growing wave of thieves targeting motorised bicycles, with thefts surging 27 per cent over two years, rising from 1644 to 2080 in the year to March.
Car thefts were among the key drivers of the state's overall crime rate, mirroring findings from the NSW Bureau of Crime Statistics and Research that showed retail theft had increased by nine per cent in the past two years.
In February, the NSW government estimated there were 760,000 e-bikes in the state following a rapid growth in sales.
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