In his new book, “Long for This World,” Jonathan Weiner profiles gerontologist Aubrey de Grey and other researchers and their work in the life-extension field: on “Methuselah mutants,” creatures that live much longer than the rest of their species; genes like Sir2 (Silent Information Regulator 2), which may be responsible for some of the life-extending effects of extremely calorie-restricted diets; and related proteins, known as sirtuins, that led to the discovery of resveratrol, a compound in the skin of grapes that can activate sirtuins and prolong a lab animal’s life.