The mother of Sim Srey Touch said her daughter used a false document that showed her age as "22 or 23" to get hired at the factory, The Phnom Penh Post reported.
Srey Touch had worked at the Wing Star Shoes factory only two weeks when the roof collapsed early Thursday, killing her and one other person. At least 11 other workers were injured, authorities said.
Three of Srey Touch's sisters also worked at the factory in Kampong Speu province, outside the capital Phnom Penh.
It was not immediately known how many were in the building at the time of the incident, but China's official Xinhua News Agency, quoting police, said more than 100 workers may have been trapped in the rubble.
The Post said police feared the death toll could rise as rescue teams, police and the prime minister's bodyguard unit worked to pull those caught under the rubble.
The report said the incident occurred about 7:30 a.m. Police said a concrete roof slab measuring about 8 meters (26 feet) by 16 meters (52 feet) collapsed and fell.
The Post said the factory makes running shoes.
Cambodian Prime Minister Hun Sen's son, Hun Many, who is a candidate in Kampong Speu in the upcoming national election, was at the scene, the report said.
"We see that dozens of heavy objects in the roof have fallen down and hit people," one witness said.
"We don't know how many are dead, how many injured but likely a lot, because authorities are pulling people out and the building is still collapsing," a rights group official said.
The New York Times quoted Ken Loo, head of the Garment Manufacturers Association in Cambodia, as saying the steel beams holding up a concrete-floored storage area at mezzanine height between two buildings gave way.
Last month, a Bangladesh garment factory complex near Dhaka collapsed, killing more than 1,120 people, raising international calls on multinational retailers to assume more responsibility for the safety of workers at their suppliers, the report said.