But wasn't he already known to be the youngest CO in starfleet?
Not canonically.
The Making of Star Trek (the source of a lot of fannish conventional wisdom) said that he was the youngest-ever captain of a starship-class vessel (i.e. a large capital ship like the
Enterprise rather than something smaller like his destroyer-ish first command). But that was probably just a handwave to reconcile Hollywood's desire for mid-thirties leading men with the fact that command of a top-of-the-line ship would usually fall to more seasoned officers. It was never actually a story point in TOS itself.
Memory Beta lists him as the Captain of the Oxford, the Lydia Sutherland and the Saladin before he came onboard the Enterprise. I don't know if any of these can be in the same continuity, but I recall Kirk (or someone lese) mentioning that he was the CO of Gary Mitchel on a ship prior to their assignment on the Enterprise in the second pilot.
Elizabeth Dehner said that Kirk "asked for him" (Gary) "on your first command." That phrasing implied that it was a previous ship, though some fans have assumed she meant the
Enterprise, because the only other mention of Kirk having a prior command was in TMoST.
Those three ships are all different tie-ins' separate conjectures about what his prior command was -- the
Saladin from DC's "First Mission" annual, the
Lydia Sutherland from Vonda McIntyre's near-contemporary but incompatible
Enterprise: The First Adventure, and the
Oxford from DC's later "Star-Crossed" storyline -- which doesn't necessarily contradict either of those, because it shows the beginning of Kirk's command of the
Oxford, while the others show the end of his tour aboard the ship in question and his transfer to the
Enterprise. So he could've commanded as many as two of the three.
I think the Kelvin movies didn't do Kirk a favor with that. I find the "real" Kirk to be a lot more interesting than the one pop culture sees.
I don't think the Kelvin movies show a single version of Kirk, though. The intent was always to show his growth from an unruly renegade
toward the gifted captain we knew from TOS. The Kirk of ST '09 was very much the womanizing-rebel stereotype of Kirk. The Kirk of STID started out that way, still short of deserving the command he'd been given, but the events of the film forced him to start growing up. And the Kirk of STB, three years into the five-year mission and about to turn 30, has essentially become the Kirk we know, the more disciplined and thoughtful captain who has a lot of self-doubt but is a capable and confident leader nonetheless, and who still has a roguish man of action not far beneath the surface. Note that STB is the only film of the three that doesn't show Kirk as a womanizer. He's all business, all about the mission.