It's strange that both Sisko and Kira kept their postings when the wormhole was discovered, but then again, after the wormhole was discovered the station was still just a port-of-call for crews to shop and drink and for ships to be repaired before moving through the wormhole. It wasn't until the Dominion became a threat that a major change in the command staff might've been called for. By then, Sisko could've been promoted to the admiralty (since his position was as much political as administrative at that point), and Kira, well, she was an officer with experience fighting the Dominion. Why move her away from the first line of defense?
At any rate, is there some reason to think that a 26-year-old can't be competent at her job?