I was not debating that the Kodos thing happened 20 years before. They say it several times in the episode. I was debating that it was intended by the writers of "The Conscience of the King" that Kirk was only 13 years old twenty years before. William Shatner was born on March 22, 1931, so he was 35 years old
when they were shooting the episode. At that point in the series timeline, they hadn't yet established a definitive age for Captain Kirk, and it's very possible that Shatner was playing Captain Kirk as his age or slightly older. So the intention may have been that Kirk was stationed on Tarsus IV as a 16 to 18 year old cadet. Later developments on the show made that impossible, of course.