"At that time, taking a gun and shooting somebody had become as easy as drinking a glass of water," Ishmael Beah, now 26, told delegates at the "Free Children from War" conference.
The two-day gathering hosted by France and the UN children's agency UNICEF is to produce a set of guidelines for agencies and governments to help the estimated 250,000 children currently fighting in wars, mostly in Africa.
"There are no excuses. If you don't do it with these children now, you are going to have bigger problems later on," warned Beah to a rapt audience at a Paris conference center.
Beah, who took part in an eight-month demobilisation programme in his home country that put him on the road back to normal childhood, pointed out failures in international efforts.
"When you rehabilitate someone and throw them back into society, it's not good enough," he said, calling for a "follow-up" and more assistance to ensure that former child soldiers do not go back to war.
"You have to find a way so th