Apple's main supplier in China, the Taiwan-based manufacturer Foxconn, has apologised for employing workers as young as 14.
In a statement, Foxconn acknowledges that some students who took part in its summer internship programme were below China's minimum legal working age of 16.
The company says the interns were employed at its factory in the city of Yantai in eastern China.
Foxconn has previously been accused of permitting poor conditions for its workers, the BBC reports.
The firm is best known for producing iPhones and iPads for Apple, but it also makes products for other companies, such as Microsoft and Hewlett-Packard.
It says it ordered an internal investigation after Chinese media reports and the US-based group China Labor Watch said it was hiring underage interns.
China Labor Watch says they were mainly sent to Foxconn by schools, who should take the main responsibility, but the company is also culpable for not confirming their ages.
Firms 'under pressure' to relax standards
The regional editor for China at the Economist Intelligence Unit, Duncan Innes-Ker, says fake IDs are only part of the problem.
"The market for relatively unskilled labour in China has been tightening," he says, "and now there is even more incentive for companies to relax their standards, so they try and fill up their work spaces.
"Companies do have a responsibility to check with the people that they are hiring are above that age. The difficulty is the enforcement of regulation, which in China has been a grey area for a number of years."
Mr Innes-Ker says he thinks Apple has been putting more pressure on Foxconn to raise its standards, "because whenever a story comes out that involves Foxconn, it's inevitable that Apple gets linked with it."