Each brand of feed should have it's own chart on the bag. They are formulated with slight differences so they have slightly different suggestions. However it's not a big deal to feed anything at a specific age. I have trouble getting different feeds here so I feed mine 22% gamebird starter from the time they hatch until they die and just put out oyster shell for the layers. There are only 2 really concrete rules. The first is not to keep feeding medicated feeds too long because it can cause a vitamin deficiency and many don't want to eat the eggs of chickens still on medicated feed. 2nd you want to avoid giving too much calcium too early to chicks which is why you feed something other than layer and don't give oyster shell until they get near laying or some wait until they see the first egg. After they start laying it's best to give a calcium supplement like oyster shell whether you feed layer or not to make sure they have all they need to make the egg shells. Some do not do that though and have healthy chickens with hard egg shells.
Anything else you feed is a treat and should not be given too often. Grit should also be provided if they are eating anything but poultry feed. Mine get whatever leftovers we don't eat and otherwise I give them a little black oil sunflower seed(boss) daily in winter. BOSS is healthier than scratch and more calories than corn so it helps hold weight and produce heat better than either of those in winter. It's also not harmful in summer.
