Big(1987)

Marco, a nine year old boy, is in love with his elementary teacher, and feels neglected by his workaholic parents. When they forget about his birthday he wishes he could grow up at once. This really happens and Marco begins a new life as an adult. He rents a room in his teacher's flat and she falls in love with him. He even fakes a kidnapping when he runs out of money but he is mistaken for the kidnapper.