Oyster shell - at what age?

[Auscal]

Firstly, a big thank you to all the regulars here who answer the same questions time & time again asked by overly concerned newbies LOL

I did search,but couldn't find an answer to this question - at what age should I add oyster shell?

I have four 14 week olds (RIR/Delaware & delaware) that are eating an 18% starter/grower crumble. I also have two eight week old (delaware & BR) on the same food (together during the day, but separated at night until they become integrated). Hopefully they should be together 24/7 within two weeks or so. I can't change the older girls over to layer, because of the youngsters, so, my plan is to keep them all on the grower/starter until the youngest are 16 weeks or so, then all onto layer. In the meantime, at what age should I make oyster shell available? - and, what is to stop the younger ones eating it (and the excess calcium causing them growth/bone problems)?

TIA advance for answers, and suggestions.

 

[Opa]

I provide oyster shell for my layers to free feed and since 14 to 15 weeks is when I integrate young birds to the laying flock they will be getting oyster shell. Chickens seem to inately know what they need with regard to calcium and grit. Since they will be laying soon the oyster shell can't be a bad thing.