The court transferred the case to the Juvenile Justice Board.
The youth was arrested for allegedly killing three of his colleagues in Natraj Bar at Vidyavihar in central suburban Mumbai last year.
Defence lawyer Swapnil Dige said they moved the sessions court saying that the accused had not completed 18 years of age at the time of offence and therefore he cannot be tried in a trial court.
"The investigation officer did not confirm the age of the accused but declared him as an adult without any proper verification," Dige said, adding even the family of the accused could not help him as they all were illiterate.
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"We called the accused's father, uncle and the school principal as witnesses. Also, his school leaving certificate, Below Poverty Line (BPL) card and panchayat certificate were provided to the court," he said.
Accepting Dige's argument, Sessions judge G T Utpat sent the case to the Juvenile Justice board.
In January last year, the bodies of watchman Bhaskar Dalvi and two waiters Basant Madoria and Raja Dakolia were found lying in the pool of blood in the Bar.
According to police, each of them was stabbed around 10-15 times.