I have a mix of age groups, and they all free range. I really can't control who eats where or what, so until they are all old enough to lay, they will be eating grower and I will offer oyster shell. My question is, will it hurt the younger chickens if they eat the oyster shell? My oldest chickens are just getting old enough to lay so I want to start offering it to them, by my other ones are 13 and 11 weeks old. Just like the food, though, I don't know how to offer it free choice to the oldest ones and not have the younger ones eat it too. Thanks!!