The high-profile case, being tried in a special fast-track court, has provoked protest, government panels, calls for reform and soul-searching on the treatment of women in India, the Los Angeles Times said.
The minor suspect will not face trial before the special court, The Times of India reported.
A magistrate announced the decision after the panel reviewed the suspect's elementary school records to prove his age.
The suspect will likely be tried in a juvenile court, the Los Angeles Times speculated Thursday, though no plans to do so were reported Monday.
The victim was a 23-year-old physiotherapy student who attended a movie with a 28-year-old male friend, then boarded what they thought was a commuter bus home. Aboard the bus, with window curtains drawn, they were beaten by assailants with metal rods, and the woman was raped so brutally she suffered internal injuries. The couple was later dumped naked along the road.
The woman died two weeks later, the newspaper reported.
Police said all six suspects confessed after their DNA was found on the victim's body, and her blood was found on the suspects' clothing, The Times of India reported.