People stopped smoking.......
LOL. I'd say antibiotics, vaccines, the end of public health scourges like typhus, diphtheria, tuberculosis and polio, far more capable emergency rooms, far better surgical techniques, significantly better treatment of chronic diseases such as heart disease and diabetes.
If you want to go back before 1945, the biggest difference in extending life was treating sewage and providing clean drinking water. That saved millions of lives over the years.