The other issue with a low metabolism is, you become more susceptible to germs and can die of an infection more easily. So much for living longer.
I think one reason endurance runners don’t usually age well has nothing to do with metabolism. They are out in the sun a lot more than most of us, and that takes a toll. People who sail a lot usually look old before their time, too.
I think there is something to fasting, but personally I’m not willing to do IF. I have a feeling you get decent effects by having just 3 meals a day so you get some longer fasting times, and also probably doing a 1-2 day fast a few times a year. Supposedly French women do that, but who knows. I agree with the others on here who think quality of life is most important, and that pretty much disappears when you’re starving.