Good luck,
@Tommy.
For most people, good health and being in shape is a lifestyle choice. It isn’t a complicated problem set, but it isn’t easy for most people. It takes time and effort every day, which means you aren’t doing something else with that time (opportunity cost), and that’s just too much for most people.
Most people view diets as something to go on for a short period of time, as opposed to something to follow their entire life. (There’s an entire industry around this that doesn’t exactly help all the time.). As such, they quickly fall out of any fad.
Additionally, when people do change their diets or start working out, once they hit their goals or see results they think they’ve arrived. You see it in professional sports teams, a lot - it’s a major reason champions don’t repeat - once they get the ring they think they are there and that’s it. They forget that all the effort that went into getting them there is what will be required to stay there. Like most pursuits, good health isn’t a static destination, it’s more a ship in a gentle current - you have to keep moving against the current or you regress.
While it isn’t all downhill after 43-45, that’s definitely where most men (women?) start to see irreversible degradation of physical ability (body breakdown). With modest effort and good discipline, though, you can change your life and enjoy the best health for which you are genetically coded.
If you haven’t seen Jimmy Johnson’s HOF induction speech, I recommend watching the whole thing, but particularly from 8:40 onward:
Live with intention. Get after it and good luck!