The Secret Exercise Trick for Flatter Abs After 40 The Secret Exercise Trick for Flatter Abs After 40 Getting a flat stomach is hard at any age. But, yes, it's harder to do after 40, when the grim reality of a declining muscle mass and a less powerful metabolism start to set in. Throw in a busy job, maybe a family, and so many other factors and responsibilities that make your days after 40 way different than your days in your 20s or 30s, and it's all too easy to let your exercise goals fall by the wayside. That being said, there still are plenty of things you can do to trim the fat around the middle and achieve the flat abs you've always wanted.