Simply put, you’re wrong. The SW 1977 shooting script (the revised fourth draft from March 1976) DOES say that Luke is 20 and Leia is about 16.

This is not to be confused with the “public version” of the shooting script, which came out in the wake of the movie’s release. This was altered after the fact to be a mirror image the actual film - a practice Lucasfilm would continue with future movies.

And even that gives Luke and Leia different ages: Luke’s age is now 18, but Leia is still “about sixteen years old.”