Manohar Lal Sharma, the lawyer for Mukesh Singh, said at a court hearing that his client is under 18, and that police documents indicating Singh is 26 had been "manipulated."
"If you could just see his face, he is only a child," Sharma told The Associated Press.
The five charged in last month's attack in New Delhi face a possible death penalty if convicted. A sixth suspect says he is 17 years old, and if he is tried in a juvenile court he would face a maximum sentence of three years in a reform facility.
The rape and brutal beating of the woman, who died of severe internal injuries, has set off an impassioned debate about what India needs to do to prevent such tragedies. Protesters and politicians have called for tougher rape laws, police reforms and a transformation in the way the country treats women.
The five men charged in the incident live in a slum in New Delhi and were allegedly riding in an off-duty bus that picked up the woman and a male companion in the evening of Dec. 16 in a posh part of the capital. Both were beaten severely and thrown off the bus. The woman died two weeks later of the injuries in a Singapore hospital.
The hearing has been closed to the media under a gag order placed by the magistrate. The defendants have been brought to the court with their faces masked, and authorities have not released photos of the accused.
Sharma said Singh had added several years to his age to get a driver's license. He said he asked the court to order a medical test to determine his client's age, but that the court did not indicate whether it would meet his request.
The case was expected to be shifted to the fast-track court Monday to avoid the delays that plague most trials in India. But the court addressed only procedural matters and paperwork at the hearing, and the case was not handed over.
The next hearing was scheduled for Thursday. It was not clear whether the case will be handed over then to the fast-track court, which was created this month to deal with crimes against women.
Last week, Sharma made a series of inflammatory and often-contradictory statements, saying that police had beaten the five charged suspects and placed other prisoners in their cells to threaten them with knives.
On Sunday, police said they had arrested six suspects in another gang rape of a bus passenger in India.
Police officer Raj Jeet Singh said a 29-year-old woman was the only passenger on a bus as she was traveling to her village in northern Punjab state on Friday night. The driver took her to a desolate location, he said. There, the driver and the conductor took her to a building where they were joined by five friends and took turns raping her throughout the night, Singh said.
The driver dropped the woman off at her village early Saturday, he said, adding that police arrested six suspects on Saturday and were searching for another.
Also on Saturday, police arrested a 32-year-old man for allegedly raping and killing a 9-year-old girl two weeks ago in Ahmednagar in western India, the Press Trust of India news agency reported.