The best food to feed guppies, both young and old is Brine shrimp and also some micro-worms. Although these are the best foods you should not feed them these all the time and should feed them other food like tubi-flex and dried flake food also. You should feed you guppies at least twice a day with the light turned on at each feeding for about one hour. I feed my guppies at around 11am with newly hatched brine shrimp, at 2pm I feed them tubi-flex or bloodworms (sometimes alive but mostly frozen) and then at 5pm I feed them dried flake food. Below is a picture of some of the foods which I feed my guppies.