The announcement comes a week after the group announced it would stop recruiting children under 17.
Farc leader Ivan Marquez told Colombia's Caracol Radio that 13 children under 15 are currently in their ranks.
He said "after internal consultation with the guerrilla structures across the country, the Farc leadership could determine there were only 13 fighters younger than 15" in its ranks”, adding they would be "discharged from the war within a short period of time".
The Colombia's Ministry of Defence says that more than 2,600 children under the age of 18 have left Farc, either fleeing the group or being captured by security forces, since 2002.
The figures suggest the majority were between 16 and 17 years of age, but almost a third were younger.
Farc is currently engaged in a peace process with the government and have reached partial agreements on issues, including the drugs trade, land reform and political participation.