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, 44 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 111 on sugar cheat. Currently fighting off sugar/candy cravings with bulletproof cocoa and a little rice.
I (try to) follow by-the-book primal as advocated by Mark Sisson, except for whey powder and a bit of cream. I advocate a two-month strict adjustment for newbies. But everybody is different and should tweak Primal to their own needs.