The police said a bone marrow test is being conducted to determine the age of the accused.
The girl, whose mother is a daily wager, was out playing with the boys on Sunday afternoon when they drugged her, the police say. The boys were from the same neighbourhood. They allegedly took the girl to an abandoned house nearby and gang-raped her through the night.
The girl was found the next day by her worried parents who came looking for her. She was taken to hospital, where initial medical reports have confirmed rape.
The girl is currently in a women's home, while the accused are being held at an observation centre in Guwahati.
The incident comes as the country debates rape laws that provide for three years in a correctional home as maximum punishment for a juvenile who has committed rape or even murder. The law defines a juvenile as below 18 years.
The debate was sparked when the youngest accused in the horrific Delhi gang-rape case was held guilty of rape and murder, like his five partners in crime, but got a sentence that many argue is not commensurate with the crime.
He was a few months short of 18 in December last year when the 23-year-old medical student was gang-raped and violated with an iron rod that led to injuries so grievous that she died 13 days later.
The adult men convicted in the case have been given sentenced to death by a trial court.
In a more recent case in Mumbai, one of the five men accused of gang-raping a young photojournalist last month pleaded that he is a minor.