I have three hens that are 18 months old and they occasionally will lay a thinner shelled egg. Sometimes this could be due to just stress. I also have 3 pullets and their egg shells are really hard. I feed all flock because I have a rooster and feed oyster shell on the side. The layer feed has a lot of calcium in it already and when I was using the layer feed I didn't serve OS very often. I will occasionally scramble some eggs and feed it to them. Two days later all the girls are laying perfect eggs. So perhaps the shell issue may be more related to protein content than calcium. Different feeds have different protein content. The all flock I use is 18% protein. I can't remember what it is in layer feed. I would have to google it.
