Shell quality is an indicator of good health and proper nutrition. Birds that have soft shells have something lacking in their diet or they are having health problems. Sometimes it happens by coincidence, and sometimes birds lay weird eggs due to age or molting. If you have bad shells, the eggs don't hatch well (if incubated) and if you are selling them for eating they will break easily and bacteria is more likely to get in the egg.
Pullets who start to lay should have nice hard shells from the start, if not then at least by the 5th egg. Some birds get enough calcium from feed, but it depends on what you feed them. I offer mine oyster shells. They can eat them if they want. I also give them hard boiled eggs with the shells so they can get calcium that way. There are also foods like strawberries that have lots of calcium and you can add those to the diet as well.