Manga Bye-Bye, Peter Pan

Never far from the future. In this world, all children are gathered up onto an island known as the "Juvenile Circle". They receive their education there until the age of 18, when they graduate and go into society as adults. A boy named "Shizu" was overcome with strange feelings of anxiety the day after his graduation. Were they directed towards becoming an adult? Does he want to remain a child? It was neither of these. Anxiety embraced Shizu. The anxiety would lead him towards the "truth" of his world.