In its eighth annual supply-chain report, Apple also said the metal tantalum, an essential component in many electronic products, had not been sourced from war zones.
Apple, the world's most valuable technology company, has been accused by labor rights groups of building profits on the back of poorly treated and underpaid workers in Asia.
Apple's latest audit found 23 underage workers at companies supplying it with components. The previous year's audit had uncovered 74 underage workers at a single supplier.
In the report, Apple said some third-party recruiters had hired young workers illegally and without the knowledge of the hiring companies.
Cupertino, Califorina-based Apple relies heavily on Asian partners, such as Taiwan's Foxconn Technology Group, for the assembly of its iPhones and iPads.
Further up the supply chain, companies must procure essential metals such as tantalum, sometimes known as 'conflict minerals' due to their sale by armed groups in the Democratic Republic of Congo,Angola and South Sudan.
Apple said its suppliers did not procure tantalum from any third-party providers linked to armed groups in various warring African countries.
"In January 2014 we confirmed that all active, identified tantalum smelters in our supply chain were verified as conflict-free by third-party auditors," Apple said in the report.
"We're pushing our suppliers of tin, tungsten and gold just as hard to use verified sources," the company said.
The latest report identified 106 facilities that did not pay night-shift workers appropriately for legal holidays, and 105 plants that did not provide sufficient social insurance.
Apple said it had identified some abuses of migrant workers and, as a result, required suppliers to reimburse foreign contract workers $3.9 million in excessive fees paid to labor brokers.
Apple's suppliers achieved an average of 95 percent compliance with its standard maximum 60-hour work week, the company said in the report.
© Thomson Reuters 2014