Scratch can be given as a treat any time, but be sure they get enough grit, from the store or from your soil type. Oyster shell is given to laying hens to help keep the shells hard. It is not needed until they reach laying age.
I use the scratch can as a training tool to get them to run to me when I want them to, but I rarely ever allow scratch till they are about 10-12 weeks old. Oyster shells are for laying age only.
I agree with speckledhen.Whenever I give my youngsters anything besides their feed, I give them a dish of free choice granite chick grit or pigeon grit which is the same. My local TSC carries chick grit.