In the movie the characters are depicted as being 4 years apart. Michael played by Rex Smith is 17 & Jessie played by Denise Miller is 13. In real life they were 8 years apart..Rex was 24 & Denise was only 16!
Rex Smith did an interview with Andy Warhol on March 20, 1981 and, when asked his age, Rex said he was 25. The official release date for Sooner or Later was March 25, 1979, which would've made Rex 23 when the movie was released, and since his birthday is in September, he would've only turned 23 in September 1979 so was probably 22-23 while filming the movie, making the age difference slightly less than noted previously.