Nick Saraev is 25 years old, far too young, it would seem, to be thinking about death. And yet, since he turned 21, he has taken steps to prevent the infirmities of old age. Every day, he takes 2,000 mg of fish oil and 4,000 IU of vitamin D to help prevent heart disease and other ailments. He steams or pressure-cooks most of his meals because, he says, charring meats creates chemicals that may increase the risk of cancer. And in the winter, he keeps the humidity of his home at 35 percent, because dry air chaps his skin and makes him cough, both of which he considers manifestations of chronic inflammation, which may be bad for longevity…