Earlier quoted context omitted.
BMI is a misleading indicator in this case. It doesn't differentiate by body composition at all (e.g. fat vs muscle). I doubt very much that adding muscle to an underweight person would decrease that person's life expectancy. Probably it would improve it (the health benefits of exercise are well known). Either way, it would improve his quality of life.
I don’t think this is true. Pretty much the only proven thing you can do if you want to live longer is calorie restriction. Supposedly this is because the fewer calories you consume the less exposure your body has to free radicals that are important in aging. The corollary of this is that building muscle, which requires eating a lot, is inferior to being a skinny rake for longevity alone. Then, if you eat barely anyt…
The one proven thing you can do to live longer is to not die.
Risk prevention / avoidance is a large part of this, but that includes a lot other than bodyweight. Seat belts, condoms, not smoking, little or no alcohol/drugs, pollution.
Several long term athlete studies show longevity, and quality of life benefits, to exercise (Stanford runners study, Scandinavian olympian study). And the specific mechanism of calorie restriction, telomere repair, is shown in both cardio and strength training.