The United Nations children's agency, UNICEF, says more than 8.4 million Syrian children have had their lives shaped by conflict.
"In Syria, violence has become commonplace, reaching homes, schools, hospitals, clinics, parks, playgrounds, places of worship," said UNICEF's regional director for the Middle East and North Africa, Peter Salama.
According to its report, UNICEF says an estimated 3.7 million Syrian children, - or one-in-three - have been born since the conflict began five years ago, including 306,000 born as refugees.
The agency says children make up half of all of Syria's refugees with more than 15,000 crossing the country's border without family.
"Five years into the war, millions of children have grown up too fast and way ahead of their time," Salama said. "Children are fighting an adult war, they are continuing to drop out of school, and many are forced into labor, while girls are marrying early."
UNICEF says that in 2015, more than half of the children recruited by armed forces and groups were under the age of 15, compared with less than 20 percent in 2014.
"These children are receiving military training and participating in combat, or taking life threatening roles at the battle-front," UNICEF says. "Parties to this conflict are using children to kill, including as executioners or snipers."
UNICEF says one of the most significant challenges to the conflict has been providing children with education as more than 2.1 million youngsters inside Syria and another 700,000 in neighboring countries are out of school.
The agency has called for $1.4 billion to provide Syria's children with learning opportunities.