MAFRAQ, JORDAN – Just 16 years old, Mohammed Hamad was heading to war. The lanky Syrian teenager was joining what United Nations officials warn might be the start of a flood of underage fighters enlisting in rebel ranks. About half of the 200 new recruits who board buses each week to Syria from Jordan’s sprawling Zaatari refugee camp are under 18, U.N. officials at the camp estimate. Hamad said it was his duty to “fight in the name of God to take back the country” from government forces. “If my generation doesn’t take up arms, the revolution will be lost,”...