The youth is irrelevant. We're dealing with space opera and thematic necessity, not with accurate military portrayals (how many times did Kirk violate the supposedly desperately important Prime Directive? Fraternize with people he simply wouldn't have had direct contact with in a realistic setting?)

Nothing in the prior episodes applies, though, because this is a different Kirk. The same BASIC characters, the same BASIC setting, but NONE of the continuity applies. Old Spock is the last lingering trace of what Used To Be, and there's no saying that anything he recalls from his prior years actually exists any more. The branch point is AT LEAST as far back as Kirk's birth, and may be farther back.