Eating less meat and more plants is going to naturally push people towards more fiber. With the added benefit of helping the environment and reducing odds of heart disease as well.

I'm of the opinion that most Americans should start supplementing fiber around 30. Earlier may be better. Also start making pro fiber dietary choices like whole wheat bread, beans as a staple ingredient (they can replace meat in a lot of dishes), breakfast choices like oatmeal, whole wheat toast, fruit, or bran flakes, and snack choices like fruit, vegetables, or popcorn.

Honestly I wish I'd gone high fiber younger. I've never had what you'd call excellent digestion, but a high fiber diet (slowly ramped up) and supplementation have brought it back to like when I was a teenager.