It has two meanings, either "you are childish" or "you keep looking young, you never get old" For example: when you see an old woman that every time you see her, she looks young in age with a baby skin, so you tell her "You never get old?" = ya 3ame ente ma btkbare?"

يا عم أنت ما بتكبر ؟ it means you have a baby face. Your look still young ^^