Kirk has been recognized by many sources as the Youngest Captain in Starfleet
...Although surprisingly, these sources do not include onscreen Star Trek itself. The only reference to a "youngest Captain" or perhaps "youngest captain" establishes that by 2364, the record is set at 25 years. Plenty of margin for somebody from the 22nd or early 23rd century to have outdone Jim Kirk.
Since Pike wore less glitter on his sleeves than Kirk did during the respective early adventures of the characters, we might argue he was a bit younger in "The Cage" than Kirk was in "Where No Man".
Or then simply less driven or otherwise slower to climb the rank ladder. We have no real idea when Pike made Captain, and whether he already held that rank in "The Cage" or merely served in the position of captain.
Basically, Pike is a blank slate, and every theory about him can be assumed to be more or less correct. Even Greenwood's take on the character offers virtually nothing new, except perhaps slightly more grey to favor the upper end of age estimates.