My memory is that all Roddenberry's treatments for Star Trek had the Captain character, whether he was named April, Pike, or Kirk as "about 34 years old." If they had chosen to mention Kirk's age in dialogue in "Where No Man . . . " or during Season One, chances are good that they would've used the "34 years old" figure there, too, so, when Peeples was writing the second pilot and Gene was polishing it, they would most likely have been thinking that Kirk was 34, and that 15 years earlier, he was 19.
Fast-forward two years to the filming of "The Deadly Years." TV characters generally exist in an "Eternal Present" and character aging is rarely acknowledged unless vital to the plot. (It's like Bronze Age Superman and Batman being "eternally 29" pre-Crisis.) So, instead of bumping Kirk's age up to 36 in "The Deadly Years," they made the "about 34" of the format documents specifically 34 in that episode (although Kirk, in his addled state, could've misspoke). They were not predicting the rise of obsessive fan chronologists sifting through conflicting esoteric trivia to come up with ironclad historical biographical sketches of the characters. They were making a weekly tv show. If the big picture held together, great; who really cared whether "the only death penalty left on the books" was General Order Seven or General Order Four? (Well, WE do, of course, but we're weird that way.)