Manga Bye-bye, Peter Pan

In this world, all underage children end up on the island, where they receive special education until the age of 18, after which they become part of society already in adulthood. The main character immediately after graduation from school feels an extremely strange feeling. Did they really have to become adults? Or could he have remained a child? The guy's suspicions and feelings will lead to the fact that he will soon discover the amazing secrets of this world.