Cusack, who was only 16, had to be emancipated from his parents to star in the 1985 movie, The Sure Thing. The producer, Roger Birnbaum, became his legal guardian once filming started.
The reason for it was that teenage actors bring in a lot of complications for filming. They have to have tutors with them all the time and can only work for 6 hours a day. That's why it's not unusual for older actors playing 16 year olds in movies.
Cusack was only 16 at the time, and wasn’t even an option at first due to his age. The casting directors continued to urge the director to let Cusack audition, and finally was allowed to. It’s said that the director felt that the had to cast Cusack after that great audition.
The Sure Thing is about two college kids going on a cross-country road trip to find their ideal match. The movie is based on an actual experience of the writer, Steven Bloom.