It is the dose

Yea I agree. It's the dose, context and recovery. (I have written more in my other replies along with the example of exercise)

woman in her 80s that looks and moves like someone in their 40s

It's definitely possible. I have seen many who climb boulders (simple ones not bouldering sport level), carry heavy loads up and down the mountains everyday well into their 80s and 90s.

They also look much younger and leaner. All on a mostly plant based diet. Whole grains, nuts, fungi, and legumes.

so what's limiting that by 40 years since we invented agriculture?

I think too much food, lack of exercise, pollution etc. The plants from modern agriculture have less nutrients and we eat very few varieties. Refined sugars and ultraprocessing of food is also up there.