Baby turtles should eat daily and as adults you can feed daily or every three days. This may be different for some species. I'm going more along the lines of sliders. If you have a baby turtle that is not eating, lethargic, dull, sometimes with swollen eyes you should see a vet as those are signs of illness/vitamin deficiency. Sometimes if there is small gravel in their aquarium they can eat it and there is a danger of blockage. If you see anything that worries you I'd head to the vet. They should be bright and lively and voracious eaters (though we're not supposed to let them get fat).