The biggest factor is likely just reduced infant mortality. Today when we say today that somebody has a life expectancy of e.g. 80 years, then it does mean you can expect people to start dropping of various causes around that age, give or take a bit because infant mortality is approaching zero.

In the past infant mortality was extremely high, with frail infants invariably ending up as dead infants. And so in the past if you looked at somebody who was 15 years old, it's very likely that he'd live to roughly the same age as somebody today. It's not like he'd just start dropping around age 40. It was more a mix of people living to 5 years old and people living to 80 years old. Take the founding fathers as a random example:

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John Adams - 91

Benjamin Franklin - 84

Alexander Hamilton - 49 (killed in a duel)

John Jay - 84

Thomas Jefferson - 83

James Madison - 85

George Washington - 67 (speculation he may have been killed by a common medical practice of the time - bloodletting)

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Interestingly enough this goes all the way back to Ancient Greek times where you may be surprised upon some brief research to see how long any Greek you know of likely lived to, in spite of having effectively zero medical knowledge and hygienic practices such as shared butt sponges (in lieu of toilet paper) at the various public toilets. The reason I more contemporary samples is because one can reasonably argue a survivorship bias in the Ancient Greeks. Would you know of Philosophartes today if he had died when he was 40? Probably, but I can't prove that. The founding fathers though were all relatively young when their names were inscribed into history.

So how do you explain that life expectancy at for instance 50 years old keep increasing? For instance in my country (CH) life expectancy at 50 years old for a man was 26y in 1980, 29.5y in 2000 and 32.5y in 2020. People keep saying that infant mortality is the explanation but this is only valide for life expectancy at birth. Life expectancy at all ages as been increasing.

Infant life expectancy describes the huge changes. We went from life expectancies in the 30s not that long ago, to life expectancies in the 70s, largely due to it. The smaller changes since are going to be from many reasons that will also often vary heavily between regions. Unfortunately I lack the knowledge to even begin to intelligently speculate about the reasons for China's improvements.

Antibiotics, vaccines, improved cancer treatment, and smoking bans have all done wonders.

Declining nutritional and increasing caloric value is probably one of many contributors to rising diabetes rates though.

No great mystery, less nutritional value per unit produce just means you have to eat more produce, it doesn't force you to be malnourished.

More doesn’t account for nutritional deficiency. If zinc is missing in a foodstuff, it requires supplementing with other foodstuffs, not eating more of the same foodstuffs.