28-3=25. Had to count twice to make sure, but yeah, this should be about right.
Though 25 isn't really old by any means. In fact Kuraki-san is actually quite close in age to the rest of the gang. Even Haruko should only be 5-7 years apart from her at most. And Mamoru is 4 years older than Haruko (Check chapter 190.5), so there's a 1-3 year gap between him and Kuraki-san, depending on how long it's been since Haruko graduated from High school (age 18, which i'm using as reference).