Summarize the following:
Oyster shells, along with other calcium boosters, are only meant for egg-laying hens. Once the hens are 18 weeks old, start purchasing oyster shells. Set the shells out in an accessible location. Within a few weeks, the hens should start laying healthy eggs.  There is no risk of chickens eating too much calcium. If they don’t need it, they won’t eat it. As long as you don’t add it to the food or water, it won’t be an issue. Feeding hens younger than 18 weeks can lead to kidney damage. They only need the calcium boost when they lay eggs. Never add the shells to the feed. Instead, set a dish inside the coop or barn. Fill the bowl full of shell powder or fragments. Your chickens will instinctively eat from the bowl when they need the extra calcium. Roosters and younger chickens will avoid it.  A rabbit feeder is another way to keep the shells available. Install it on a wall where the chickens gather. You could also sprinkle some powder on the coop floor or near some plants or grit they peck. They will discover it on their own, but you may find it difficult to monitor how much of it they eat. Even if you don’t get eggs from your chickens during part of the year, stock the bowl or feeder. Hens still benefit from the calcium. It keeps their bones strong and prepares them to lay more eggs later in the year. If they need it, they will go and eat some. The most recognizable sign of calcium deficiency is thin-shelled eggs. If you notice splattered eggs in the coop or if the eggs break in your hands, calcium deficiency may be the reason. Set out more shells as needed. Make sure they are broken down and placed where the chickens can find them.  Rough spots and wrinkles on the shell are another sign of low calcium. Your chicken’s eggs should look like they belong in a supermarket. Commercial-grade feed does contain calcium. Usually this is enough, but not always. Foraging or table-scrap-fed chickens often need the extra calcium source.

Summary:
Start feeding oyster shells to hens at 18 weeks of age. Set the shells out in a feed container inside the coop. Provide oyster shells year-round. Give your hens more oyster shells if their eggs break easily.