> Why is it so bloody hard
It takes 100 years to get one data-point. For us to track a person's habit throughout their life that eventually ends up living to 100 years is not an easy or quick task.
And even if you had the best tracking in place, isolating cause and effect given how so many variables are involved is no easy task either.
So instead we have to rely on small experiments, low sample size studies, and studies where variables are hard to isolate.
But there's a lot that's known that's pretty clear, exercice is good, cardio or strength or flexibility, they all seem good. We don't know how much of it is needed, but more doesn't seem to hurt.
Having lower body fat, eating at a slight deficit, that's pretty clear as well.
Cutting out bad nutrients, chemicals, processed foods, etc. That's not totally proven, but at least it helps with weight control, and it definitely doesn't seem to be beneficial, so at best it's neutral at worst harmful.
Pollution of all kinds has negative effects. That's pretty well established. Nothing much you can do about it since about everywhere and everything is polluted it seems nowadays.
Having low stress and good nights of sleep, that seems pretty well established as well.
Genetics, ya genetics for sure. But can't do anything about it so no reason to worry about it.
Now eating good foods also help. There's ton of micro-nutrients with small studies that show benefits. So just make sure you eat plenty of all kind of good micro-nutrients. You'll get the most out of vegetables, legumes, fruits, herbs, seeds, nuts, and all that. Fatty fish are good, and other fish and seafood have some good micro-nutrients as well. Except the larger the fish, the more pollutant it contains, so don't exaggerate either on fish.
Meats are uncertain. At least keep them on the lower side, especially red meat since there might be links with some form of cancers. That said, if you do eat meat, make sure it's grass fed and all, cause meet from grass fed animals have way more good micro-nutrients in them.
Also seems you want to keep your brain active as well, continue to learn new things, helps prevent dementia and other brain degenerative issues.
Apparently good company, having people to laugh and socialize with is good as well, but unsure how much, again it clearly doesn't hurt and might be good.
That's it, but, the sad part of all this, all those things just help a little bit, mostly with quality of life and not that much with longetivity. What has helped the most with longetivity has been modern medicine and surgeries, which is pretty much the single reason why people live longer now then before.
Edit: Oh and it also seems that there are other things that can affect you a lot, like which bacteria and viruses you've been exposed too thorought your life at different stages. Exposure to these can be good or bad depending.
I also forgot good amount of fibers can prevent diverticulitis and other possible digestive issues. Proper levels of vitamins (best if obtained from foods), including D (which is best obtained from the sun). Good hydration (helps some of your organs function). And amortize sugar intake, so you don't spike your blood sugars.