Some of us never switch to Layer. We just feed Grower, Flock Raiser, something like that and offer oyster shell on the side. That way it doesn't matter how old any of them are.
By the way, if all they eat is Layer, they should not need oyster shell. If all they eat is Layer, they should get enough calcium from that for the egg shells. it's formulated to provide all they need. I certainly would not mix it. If they don't need it, why try to force them to eat too much. Too much of a good thing is often not a good thing.
Whether they forage or get low-calcium treats or just eat nothing but Layer, offering oyster shell on the side is not a bad thing. They pretty much self-regulate. But mixing oyster shell with Layer? No, I refuse to even consider that.
You can offer it when you see the first egg. You can offer it at 18 weeks. You could offer it now. If they don't need it, they won't eat enough to harm themselves. I have it available at all times and I normally have growing chicks in with the flock. You will probably see a few pecking at it, mostly out of curiosity. I don't worry about that.
I understand about Layer being cheaper. I suggest you switch when your youngest are about 18 weeks old, right after the current bag runs out.