It boils down to inborn physiological health and lifestyle. Inborn in the sense that IQ is highly predictive of lifespan, because IQ highly correlates with overall physiological health. Singaporeans live a long time because Singapore is a genetic IQ meatgrinder. High IQ people move there and have zero kids.
Americans would clearly live a lot longer than they do if they spent less on healthcare. Healthcare is currently about 20% of USA GDP and we do no better than Mexico, which spends less than 9% of GDP on a much lower cost basis. Ergo, Americans could work four days a week, renounce health care, and live much longer than they do right now.