Entertainer W.C. Fields is often credited with first saying, "Never work with children or animals" because of their unpredictability and ability to upstage anyone else. Kids in showbiz also require on-set tutors and chaperones, and can only work a certain number of hours a day. With such constraints, it almost makes sense to have adults playing much younger characters — to skirt the restrictions of child actors, but also to control when and how they "grow up." When you hire someone post-puberty, there's less risk of an unexpected growth spurt or someone's voice radically changing. Having adult actors play teenagers isn't a new concept, as some actors included on this list starred in movies and sitcoms of the 1970s. However, it does seem that lately, many high schoolers are portrayed by people who are old enough to have graduated from college. Ahead, see which of-age actors have played on-screen youngsters and just how old they were while filming these iconic teen characters.