And no matter what your age or size, you can do it. So, doing endless reps won't do a lot. The most efficient is the crunch lying on the ground. Your abs curl your spine forward, so you need to do that I don't do a lot of reps or a lot of sets. I just do really concentrated movements and really feel them.
Nutrition comes into it.
I used to do an hour of ab work when I was younger and saw no results. Now I do less and reduce calories and body fat, they show through. You can't reduce the fat in certain areas by exercising that area. You lose fat all over the body with exercise. Reduce the carbohydrates and you lose water weight.