I'm 23, 165cm, 163lbs, about 15-16% body fat (according to this body composition machine at the gym, whatever that's worth).
I'm not saying I can do endless numbers of situps or whatever, I can't (despite having quite a hard stomach). Thing is that I reach failure after a couple of sets of 20 crunches and similar stomach exercises mentioned above, yet I never feel anything the next day.
I should add I've improved at doing crunches etc and I feel my stomach getting stronger. Just I never feel sore (yes, I didn't mean "hurt").