For the most part, ab work isn't to give you a six pack. That's 90% fat loss.
Ab work holds in your guts and strengthens your core. It helps you hold yourself in form when you're doing curls or squats. Combined with lower back work it prevents huge amounts of back/hip injuries.
So the answer is, as above, yesterday. Everyone should work their core. For posture, for health, for injury prevention, if not anything else.