Pullets will lay a soft shelled egg and older hens will lay thin shelled eggs. The older hens diet is usually, in my experience, the problem. I get thin shelled eggs when I get hens that do not eat the laying pellets enough. This is usually the result of them stuffing themselves on corn or scratch. Those are fine in moderation, but not as a steady diet. Even with the laying feed (pellets) I allow them free choice, I give them oyster shell free choice. It seems to last a long time, but the shells will almost be so hard they do not break easy when I try to crack them. Then once in a while I'll get an egg that cracks when it is laid. When this happens, I cut them all back on scratch, which is really intended for treats not as a laying feed. I'm taking care of my DIL's hens right now and she was feeding her hens only scratch and corn. At first I had some cracked eggs, but now, on my feeding method, their shells are much harder, with no more problems of cracking when laid. If the hens are totally on free range and there is actually grass and bugs out there for them, they do the best with little to no problems like this. But, it is winter here and not much for them to eat, so they do need a complete feed to supplement their diet. I feed mine year round, as I don't free range mine.