Big (1988)

Twelve-year-old Josh Baskin is humiliated at a carnival when he is too small to go on a ride. He makes a wish to be big and wakes up the next morning in the body of an adult. Now he must try to make his own way in the adult world, getting a job and navigating the complexities of romance. As he learns to balance the responsibilities of adulthood with the exuberance of youth, he realizes the importance of maintaining a youthful spirit. In the end, he finds a way to undo his wish and return to his mother.