The Young World (used book) - Chris Weitz
Welcome to New York, a city ruled by teens.
After a mysterious Sickness wipes out the rest of the population, the young survivors assemble into tightly run tribes. Jefferson, the reluctant leader of the Washington Square tribe, and Donna, the girl he's secretly in love with, have carved out a precarious existence among the chaos.
But when a fellow tribe member discovers a clue that may hold the cure for the Sickness, five teens set out on a life-altering road trip, exchanging gunfire with enemy gangs, escaping cults and militias, braving the wilds of the subway - all in order to save humankind.