Chicken oyster shell and grit for every age and stage.
When's the last time you saw a chicken tooth?
Grit takes the place of what teeth do... it grinds and crushes the food in the gizzards. Your birds should always have some on hand. Chickens will self-regulate their grit intake, so no worries about overdoing it. But if they need some and can't get to any, they may end up with digestive problems.