Ralph Macchio was 22 during filming. Some of the cast did not believe him when they asked about his age.
The yellow classic automobile that Daniel polishes in the famous "wax-on/wax-off" training scene, then later offered by Mr. Miyagi as Daniel's birthday gift, was actually given to Ralph Macchio by the producer, and he still owns it. The car is a 1948 Ford Super De Luxe.
The studio wanted to drop the "drunk Miyagi" scene, feeling it slowed the pace down. Director John G. Avildsen argued for it and has felt it was the scene that got Pat Morita nominated for an Oscar.
The idea for The Karate Kid film came from producer Jerry Weintraub, who heard a news report in 1983 about a scrawny teen being bullied who became a martial artist.