Terok Nor is old and lived a hard life. I imagine its structure is riddled with micro fractures. You can only maintain and refurbish such structure defects so much, take it from an aerospace engineer.
Deep Space 9 isn't that old. Construction began in 2346 and finished in 2351 -- it's only about 37 years old as of the end of Plagues of Night
. By contrast, numerous Federation starships and starbases are actually much older than Deep Space 9 -- the entire run of Excelsior
- and Miranda
-class starships, for instance.
True. But is also doing work it was not designed to do. I don't think it was meant to be caught in front line battles, endure the gravitation loads of a nearby wormhole, or even the high volume of docking activity it experienced when it went from an ore processing station to a military and commercial outpost.