Lifespan extension has been observed in studies which have restricted either calorie intake or protein intake or only methionine intake.
I have not seen any studies that clarify whether calorie restriction has any effects beyond those caused by the protein restriction included in the calorie-restricted diet and whether protein restriction has any effects beyond those caused by the methionine restriction inherent to a protein-restricted diet.
Methionine is required for the initiation of any protein synthesis, so presumably when it is deficient the global rate of protein synthesis in the body is lowered, which is what might be the cause of the extension of the lifespan.
However this may cause problems when a high rate of protein synthesis is needed, e.g. in cases of diseases or accidents.
Maybe for mice living in safe little cages where they don't have to worry about falling down the stairs and breaking a hip. Telling humans to restrict protein intake is idiotic, bordering on irresponsible. Sarcopenia is a huge risk factor for the elderly. Lack of lean muscle mass is a risk factor for metabolic disease (type-2 diabetes) and disabling falls. We need a large amount of protein (including the right mix of essential amino acids) to build and maintain muscle. And as we age the digestive system becomes less effective at absorbing protein, so we should increase consumption a bit as we age.
I mean actually extending it. For example if the natural lifespan is 3 years, can we now reliably get 4 or 5 years?