That's the question. Actually, it is not clear that calorie restriction increases human lifespans. It works really well with fruit flies and rodents, but monkey results are mixed. I don't know if there is any direct evidence of calorie restriction increasing lifespans in humans. Actually, practicing calorie restriction when young can affect development and practicing it when old might also be dangerous due to low BMI or bone loss. And if muscle is independently associated with mortality in men then losing muscle due to controlled starvation might not be helpful either.