At peace talks in Cuba, the leftist guerilla group had already pledged to stop recruiting fighters under 17, but the Colombian government said that did not go far enough.
International authorities and rights groups define any member of a force aged under 18 as a child soldier.
"Today we communicate to the country our decision to end the recruitment of those aged under 18 to the FARC," a negotiator for the Marxist rebel force, Victoria Andino, said, reading out a statement.
Today's announcement appeared to be a gesture to ease negotiations by the FARC, whose leaders have warned in recent weeks of lingering obstacles to a peace deal.
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The two sides have signed deals on four of the six agenda items at the talks: justice for victims, land reform, a political role for ex-rebels and fighting the drug trafficking that fuels the conflict.
The unsettled issues are disarmament and the mechanism by which the final accord will be ratified.
Colombia's President Juan Manuel Santos said Monday a referendum would be held on a possible peace deal whether the rebels want a vote or not.