The Knocked Up director penned the script at the start of his career, before finding success in Hollywood and becoming one of the top kings of comedy.
The episode, in which Homer Simpson is hypnotised into thinking he is 10 years old, was submitted during the first season of the animated sitcom, which began broadcasting in December 1989.
Apatow tells Conan O'Brien on his Serious Jibber-Jabber internet show, "I got a call from The Simpsons saying, 'We're going to shoot that episode next year.' Now I can retire. (The script was) the first thing I ever wrote."