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.

But it is more expensive to be on medication from eating shitty food then it is to eat more fiber and healthy foods. Fermented foods help your gut microbiome like crazy.

It's a lot easier not to start smoking than to never start eating unhealthy. Especially since not many smokers hand their kids a pack from a young age