On this thread, you said you were a 15-year-old male.
good god, dude, get a grip.
You should be stuffing your face with protein and good fats and veggies and probably some white rice.
NOT fasting for "longevity" or any other reason for that matter.
If you deprive your body of nutrition while you're still growing, you'll probably cut your longevity short for some other reason.
Enjoy your youth,
And don't think of fasting until you're 30 or 30 pounds overweight (not likely), whichever comes later.
5'0" female, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.