A character lies about their age or hides their real age in order to get around a social or legal boundary. The reasons the character does this vary. Sometimes they feel a sense of duty and are stopped by that age boundary. Sometimes they feel too limited by their age and wish to hide it. Maybe they feel that they are above the law. Either way, the interest of the situation comes either from the character trying to keep the secret until they almost inevitably get caught, or sometimes in Rule of Funny situations, it comes from The Reveal and consequences of that reveal.