The leftist guerrillas' use of underage fighters was one of the thorniest issues in four years of peace talks with the government, which yielded an accord last November to end half a century of conflict.
"The first humanitarian operation of 2017 is now under way, with minors leaving the Transitional Normalization Zones," the camps where FARC members are currently disarming, said the International Committee of the Red Cross.
The FARC, or Revolutionary Armed Forces of Colombia, have gathered at these specially designated zones to disarm. They began the process of turning over their weapons Wednesday under UN supervision.
Under the peace deal, the FARC are due to remove all guerrillas under the age of 15 from their ranks.
But the process is running behind schedule. Just 13 child rebels have returned to civilian life so far, according to the government.
It did not give an estimate of how many remain in the FARC's ranks. In May 2016, the defense minister gave a figure of 170, out of a total of some 7,000 guerrillas.
The FARC disputed that figure, saying just 23 of its members were under the age of 15.
President Juan Manuel Santos is seeking to end a messy, multi-sided conflict that has killed more than 260,000 people and left 60,000 missing.
The government is currently holding separate talks with the country's last active rebel group, the National Liberation Army (ELN).
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