Flat stomachs are possible but it takes a lot of time and effort and a massive loss of body fat. It requires like 7-8% body fat to be flat.
Uh, what? You can see abs at 20, solid definition by 15, and shredded by 10. Of course it varies from person to person with a big contributor to being how muscular you are, but you by no means need to be bordering body building percentages to attain flatness.