Layer feed is calcium enriched to support the hen in the production of eggs -- for birds not in production (ie too young) the excess calcium can have serious health effects. It is best not to feed until at least 18 weeks of age or, better still, until all the birds are actively producing eggs. If you opt to go with the former you can simply offer oyster shell in a separate dish for those birds that *are* laying and continue to feed grower/starter rations to the whole flock - switching to layer once the last bird begins to lay....or even continue with that feed plan even after all birds are laying.