You don't necessarily have to supply oyster shell at all. If all you feed is Layer when they start to lay, it should have enough extra calcium in it for the egg shells. But there is nothing wrong with offering oyster shell on the side once they are laying age. I do and just consider it good practice. If they need the oyster shell for their eggs, they'll eat some. If they don't need it, they pretty much leave it alone and it lasts for a very long time.
Studies have shown that it is harmful to feed a lot of calcium to growing chicks. I don't know what age you can safely switch to extra calcium. Those studies don't get into that. Many of us offer oyster shell on the side when we have mixed age flocks so the hens that are laying can get extra calcium and the ones that don't need it don't eat it. Offer it on the side. Don't mix it with the feed.
You could offer oyster shell on the side now and it will almost certainly do no harm. But I'd suggest waiting until they start to lay. They simply don't need it now.
I hope I'm not taking over your thread, but i've always curious about oyster shells - they seem to be pretty sharp, big and well, to me, quite "dangerous" for chickens to just gobble it down. My chickens hardly eat them - I do offer oyster shell on a side. Does anyone have an explanation for why these slightly big oyster shells are ok for laying chickens to eat?
Do not begin oyster shells until they start to lay. If you feed them layer feed (starting about week 18) they will have all the calcium they need and oyster shells are superfluous if not damaging.
Oyster shells are almost pure calcium and the digestive system in a chicken grinds it up and dissolves the minerals.
As you know, chickens don't have teeth, so the gizzard will hold small stones (grit) and even some oyster sells (until all the calcium is absorbed and the shell has disintegrated.) At least I understand it that way. How else could they grind up those grasshoppers?
Since the chicken can choose to eat or not eat any oyster shells that you would make available "free choice"-- leaving oyster shell out should do no harm to chickens, the chicks would just ignore it until they needed it. IMO