The extra calcium in layer can be bad for young chicks. It can damage their liver. It is not that their liver is going to explode the first time they take a bite of oyster shell, but over time, it can cause damage. I don't know what the right age is that extra calcium does not hurt them. They don't need it until they are laying, so that is one consideration. The university and extension websites recommend waiting until they are 20 weeks or start laying and these are people that have studied chickens in great detail, but that recommendation may be more economic than health related. Why spend money feeding oyster shell if it is not needed.
I've used the16% grower with oyster shell on the side and it worked out OK. In my opinion, it is the best I can do.
Good luck!