Wait 'Til You're Older (2005)

Kwong is a young boy coping with a difficult family life. His mother committed suicide when she discovered that her husband, Chan, was having an affair. Since her death, Chan has married his mistress, Tsui, whom Kwong understandably hates. He wishes that he could become an adult so he wouldn't have to do what she says anymore - and, as luck would have it, Kwong has a run-in with a strange drifter in the park who has developed a magical growth formula. Kwong steals it, which soon causes him to begin aging very rapidly. Now as an adult, Kwong can do anything he wants - like stay out late, get drunk or even flirt with his sexy teacher, Miss Lee - but he doesn't stop growing, and within a couple of days Kwong is to become an old man.