This site specifically mentions feeding.
Oregon State Feeding Chickens
http://extension.oregonstate.edu/catalog/html/pnw/pnw477/#anchor1132074
After 6 weeks switch to grower. At 20 weeks or when you see the first egg, switch to layer. I feed the combination starter/grower as that is what my local
TSC carries.
People do different things with grit. I personally believe grit, in the form of sand, should be offered to the chicks within the first week. I think it helps get their digestive system working the way it should. If all you feed is chick starter, they do not need grit. If they eat anything other than chick starter, bugs that wander in their area, any treats, wood shavings or other bedding, then they should have grit to help get the stuff through their digestive system. OK, for plain yogurt I'll conceed they probaly don't need grit, but, in general, they need grit for any treats.
You did not mention oyster shell or extra calcium. Free choice oyster shell after they start laying is never a bad choice. Don't feed it before they start laying as it can be bad for growing chicks. Layer feed contains calcium and they get calcium if they are allowed outside, either from hard shelled bugs or some rocks they use for grit like limestone. But if the egg shells are thin or are rough, extra calcium is probably required.
I personally do not mix oyster shell or grit with their feed. I figure if they are getting all they need, why force them to eat more than they need? Many people do mix it and are quite successful. Just one of my personal quirks.
