Dawson Casting is when teenager characters are portrayed on screen by people who are not teenagers.
People often criticize how common it is, but there are many good reasons why it happens.
Teenagers are still growing, and therefore, their appearances can be very fickle; a cute 14-year-old could become an awkward 15-year-old, and that can be very jarring to the audience. Also, another issue is child labor laws; by law, teenagers can only work a so many hours, and that can make it hard to finish projects on time. Avoiding having to deal with stage parents is also a huge factor. Other factors could be that adult performers are often more skilled and experienced at acting; teenagers may find the pressures of being in a TV show too much to deal with and leave after only one season.
This trope is named after the show Dawson’s Creek. Most of the cast was too old to be in school, when filming began. This trope has also been called “The 90210 Effect,” named after Beverly Hills, 90210, where the teenage characters were mostly played by adults in the their 20’s.