Chickens not actively laying eggs will get what they need for a good all-flock or chick feed, without extra calcium. It's when the pullet starts laying eggs that her need for calcium increases dramatically, and the extra oyster shell is necessary. It does no harm to have that dish of oyster shell out there sooner, the birds will try it, and not really eat very much until they are laying eggs.
Overdosing calcium is bad news! Feed balanced diets, have the oyster shell available, and it will be fine. Adding some egg shell is okay, but won't meet their needs if it's all you offer. The oyster shell is necessary.
Birds fed layer feed may also eat some oyster shell, but will eat less of it.
Mary