Ever watch Top Gun? Only the best of the best, etc. The military just loves to rank their officers from top to bottom. They fast-track the best; they fire the worst.![]()
Well, they certainly promoted Kirk fast, for him to be given command of one of 12 Starships in his early thirties. Most Starship captains that we saw in TOS looked "seasoned", about a decade or more older. (this assuming that humans' ages appear as they do now and I think that's probably not true. So they could be somewhat older. Kirk appeared to have been super "fast tracked".
It could be Spock should have been promoted somewhat earlier, if he became "the best first officer in the Fleet" after only about two years as exec (and holding down the science officer duties too). A quick rise, given there were 11 other first officers on active duty Starships, most probably having more time in the position. As others have suggested, Starfleet was perhaps uncomfortable with a Vulcan in a top command position on a human ship, so he might have been well versed in the exec duties for some time, even if he didn't officially hold that position. And perhaps Spock was unwilling to take the position, not wanting command. Perhaps preferring his science officer duties (and he certainly kept them after becoming First Officer). Perhaps he refused or dodged the position until he had seniority enough that it was a case of him accepting the position or being forced to resign or be reassigned. If he's qualified, has the efficiency ratings, has the time served and is due for the promotion and refuses it, Starfleet could think it might cause problems to have an over qualified officer in a subordinate position. Maybe they gave him a choice to accept the promotion or reconsider his position. It's interesting to consider. Not sure if Spock was fast tracked or if he was "force" tracked, so to speak, to be exec. And that's why he became the best Exec in the fleet after only a couple of years in the position. Just speculation. Maybe it happened that way, maybe not.